I've found by e-mailing it to yourself changes the pictuere to a size that most forums will accept.
If you click on the picture then look in the column on your left it will say "e-mail this file" click on that, then a box comes up asking you if you want to re-size the picture, say yes then e-mail it to yourself.
This is how I get round it (not got many computer skills I'm afraid!)