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

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