A screen টুপি is a ছবি from a movie অথবা TV show, whereas a ছবি is like just a ছবি of the celebrity/band/whatever from a ছবি shoot অথবা public appearance.
I think a screencap is when আপনি just take whatever is on the screen and make it into a picture. I mean like for a screencap you'll literally take your camera and snap a pic of the screen, অথবা you'll hit ALT+PrtScn to make a টুপি of your computer screen. Plus, screencaps are of the entire screen, normally. No cropping অথবা whatever.
I think ছবি are আরো edited and planned, rather than a যেভাবে খুশী pic of a যেভাবে খুশী scene.
