When I go to change my picture, it won't let me...

It'll let me remove it, but it won't let me upload anything different than what I already had...

Are you trying to upload from your computer? If so it can come up with that error message.

Try using either Photobucket or tiny pic to upload the picture then paste the link to the URL bar thing. O.o;

It could be the size of the picture or the type of file its saved as.
Again if your uploading from your computer, try what elle suggested.