[JeansJoe]

you can try a few different things.

if only she has the problem - clear cache files.
Upload a different image as user image see if that works. If not upload the image in the CP
Then upload the original user image again. see if that works. If not then have her recode the image with photoshop or sth. Just load the image and then safe it again as gif or whatever. This way you might remove corrupted bits in the image.
You can also Screenshot the image and then safe it again That would be really foolproof.

oh and of course recheck the dimensions of the image and what your forum allows...