Here I will post Comparison Screencaptures from 8 different versions of The Little Mermaid:

The 1990 Walt Disney Classics VHS.
The 1990 Walt Disney Classics Laserdisc.
The 1998 Walt Disney Masterpiece Collection VHS.
The 1998 Walt Disney Masterpiece Collection Laserdisc.
The 1999 Walt Disney Limited Issue DVD.
The 2006 Walt Disney Platinum Edition DVD.
The 2013 Walt Disney Diamond Edition Blu-Ray.
The 2019 Walt Disney Signature Collection Blu-Ray.

FIRST IMAGE: Walt Disney Classics VHS.
Released: May 18, 1990
Aspect Ratio: 4:3
Image Quality: I used to have comparison screencaps from an old danish VHS, but I got a copy of an VHS from the UK where the image is greater than my danish VHS. The colors are not faded at all in this VHS. You can see it in almost every screencapture, but especially in the sceencapture where Vanessa emerges from the wedding cake. Unlike the 1998 VHS, Laserdisc and the 1999 Limited Issue screencaps where the colors are too bright and faded.

SECOND IMAGE: Walt Disney Classics Laserdisc.
Released: September 21, 1990
Aspect Ratio: 4:3. This transfer shows a bit more top and bottom of the image. You can see this clearly in the screencapture where Scuttle yells in the face of Vanessa. There you can see a little bit of her wedding dress that ain't visible in the other screencaptures.
Image Quality: Just as colorful as the VHS, with some strong colors. I had the movie converted from NTSC to PAL, resulting in some frames that aren't present in various comparison caps. But it's still a great transfer.

THIRD IMAGE: Walt Disney Masterpiece Collection VHS.
Released: March 31, 1998
Aspect Ratio: 4:3. This time, however, it shows a bit more image as you can see in the screencapture where Princess Ariel swims down to grab her bag before Glut the Shark gets it. Or the screencapture where a pelican drops the lobsters on Vanessa. Here you can actually see the lobster that grabs her hair.
Image Quality: On the cover for the 1998 Masterpiece Collection VHS, Disney states that the movie is "FULLY RESTORED" and in s press release for the 1997 theatrical re-release, Disney wrote that the movie had been "digitally restored so that it looks and sounds better than ever before." This was the first time that the entire movie was digitized. It it also a great print, but it's way too bright. Look again, at the screencapture of Vanessa emerging from the wedding cake. In the last two releases, you can see the gray of the cake on and around Vanessa. Not here. It's all white, blending in with her white wedding dress. Or the screencapture of the pelican, Vanessa and the lobsters. You can''t see the blue water around her dress.

FORTH IMAGE: Walt Disney Masterpiece Collection Laserdisc.
Released: November 3, 1998.
Aspect Ratio: 1.66:1 This is the first release to feature the movie in 1.66:1, showing all the image.
Image Quality: The colors are a bit muted in comparison with the DVD. But again the colors are too bright. The lines are strong, a bit better than the VHS.

FIFTH IMAGE: Walt Disney Limited Issue DVD.
Released: December 7, 1999.
Aspect Ratio: 1.66:1 Non-Anamorphic Widescreen
Image Quality: One of the first DVD's that Disney released under the Limited Issue Banner, this version of The Little Mermaid is having special features that includes Dolby Digital 5.1 Audio and Full Color Character Artwork On Disc.
The feature appears in its original aspect ratio of approximately 1.66:1. This is not an anamorphic transfer – it has not been enhanced for 16x9 televisions. Colors and sharpness look relatively good, but the image suffers from being quite grainy, and it can get a little severe at times. This is by no means the finest presentation possible, but it’s more forgivable when considering it’s an early Disney DVD.

SIXTH IMAGE: Walt Disney Platinum Edition DVD.
Released: October 3, 2006
Aspect Ratio: 1.78:1 Anamorphic Widescreen
Image Quality: Like Aladdin and The Lion King later did, The Little Mermaid inspired a few urban legends after its home video release enabled close and countless rewatchings. One of these involved Mermaid's poster artwork which was translated to the video cover. Complaints over an offensive spire - that somehow resembled a not-to-be-exposed body part more than the other spires - led to an artwork alteration for later versions. Obviously, that is a non-issue here, since, in the Platinum tradition, neither the poster art nor the video cover is replicated for the DVD package.
The other urban legend is along the same lines, but, like the sources of the Aladdin and Lion King "controversies", it has unfortunately been addressed and "corrected" here. Those who objected to the bent knee of the priest as Eric and Vanessa walked down the aisle to wed can rest easy knowing that even if it wasn't supposed to represent an erect version of the same body part above, no one will make that mistake again. (Unless they have the Limited Issue DVD or one of the millions of copies of videocassettes and laserdiscs that were released over the past 16½ years.) The presiding clergyman's knees remain bent in subsequent shots, where they are clearly knees. But the lone kneecap's bouncy, under-the-tunic appearance during the aisle walk has been unquestionably edited out; his tiny legs are now unbent. Extremely minor though this may be (then why do it?), it does affect continuity, as the diminutive, elderly minister (who is dressed as a bishop) now is presumed to change his posture (and height) to have his knees subsequently bent.
Platinum Editions always boast drastic digital restoration work and have almost always delivered on the promise for an excellent feature presentation. Does The Little Mermaid continue that tradition and live up to those high expectations? Yes, absolutely.
The new transfer here, which offers the film in 1.78:1 anamorphic widescreen, is a clear improvement over the Limited Issue's 1.66:1 non-anamorphic widescreen exhibition, which itself was one of the less objectionable debuts. The softness, grain, and occasional ringing which marked the Limited Issue are practically bid farewell to here, as the Platinum's presentation offers rich colors, a solid picture, clearer lines, and more vibrancy. The difference in aspect ratios is minute, as this new transfer loses a few pixels in height but adds a few pixels in width.
Also, Disney made a few changes to this edition of Mermaid, as you can see in the first 8 images below.

SEVENTH IMAGE: Walt Disney Diamond Edition Blu-Ray.
Released: October 1, 2013.
Aspect Ratio: 1.78:1
Image Quality: In 2013, The Little Mermaid was given a brand new "state-of-the-art" digitally restoration, prepared for the first time in 3D! This restoration, I consider to be the most faithful to the movie. The undersea palette is lovely, with strong primary presence, satisfying black levels, and nicely balanced contrast. Detail is excellent as well. Lines are clean and refined, background textures are nicely resolved, a faint, filmic veneer of grain reveals just how small a role any egregious noise reduction played in the restoration, and virtually everything the animators brought to life in 1989 is, indeed, alive and well in 2013. Moreover, macroblocking, banding, errant noise and other troubling anomalies are either kept to a bare minimum or altogether absent. The colors are great in this version. However, there are a few changes (again) in this Diamond Edition. The first: the opening titles are slightly mistimed (which wouldn't be on my shortlist if the filmmakers didn't specifically comment on the precise timing of the opening titles and music in the disc's audio commentary). The second: a strange bar appears (for the briefest of moments) at the upper right-hand side of the screen when Ariel and Eric leap from atop giant Ursula's crown. I have made the comparesion screencaps below. But I do think this is the version closest to how the movie should look.

EIGHTH IMAGE: Walt Disney Signature Edition Blu-Ray.
Released: February 26, 2019.
Aspect Ratio: 1.78:1
Image Quality: For this "Signature Collection" Blu-ray release, Disney presents The Little Mermaid with a new transfer rather than recycling the Diamond Edition's 1080p presentation. The differences between this disc and the old one are not drastic. The UHD, of course, delivers a significantly more robust HDR palette and modestly more crisp textures; it's the way to go. The images here might seem rather dark, but on a tv, it looks really good. One thing I did notice on this transfer, is that there seems to be added a good amount of DNR, which is a shame. The gain is gone, but it also leaves the image rather soft in some shorts. But still, it's a great transfer.