Um, why won't my photo-editing thingy resize my photo?

I want to জমা করুন a picture to deviantART, but it's a somewhat big-ish file. I need a preview, apparently. But my photo-editing device things (both প্রিভিউ which comes with my Mac and the one that comes with my HP printer) will not allow me to resize my ছবি (it's scanned into the computer). Why? It's really annoying me T_T