How are you trying to upload? Do you use iPhoto? If so, select your photo from within iPhoto, choose the File>Export function, choose the size you want and then export it to the desktop. You should be able to upload from there. By the way, stay out of the iPhoto library in finder -- no good can come of mucking about in there.
If that does not work for you, give us more information about exactly what you are doing / want to do, and we'll try to help some more.