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

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