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

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