you must have a pretty big pgoto size, I have seen some large photos posted on here that I had to edit to be viewable.
When you upload, it should tell you the maximum photo size.
If not, the easiest and fastest way to decrease the file size is to right click on the photo and then send to email. It should ask you if you want to reduce the file size, hit yes, then save them from the email that pops up. I would save them with a different title so you don't loose the original if you intend to do further modifications to it.
Try that, if it doesn't work let me know.