OT: Fixing a digital image (higher quality?) HELP

Is there a way to improve the quality of a digital image after it has already been taken? I have a digital image of DD that is just beautiful, however I had to crop it from a larger image, thus lowering the quality. I want to get it printed, but Shutterfly says not to print it larger than a 4X6. Is there any way to improve it so I can print say a 5 x 7? I don't have PS or anything...but if there is a way and someone wonderful has a photo prgram that can do this and would like to help (if this is even possible!) please let me know!

Imperia