Your picture is probably too big. There is a chart underneath that shows you the largest size it can upload. If your picture is jpg, for example, it says the largest number that it can load. If yours is larger than this, it won't work. There are free editing programs that can make it smaller for you, but I'm not sure of their names.
When you do get it to work, don't be surprised when it says 'close this window'. I thought if I did, I would lose everything, but it will still be there.