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

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