Alternately, if you can't figure out how to resize them, you can upload them to Photobucket, and have it save them as message board size- 640x480. Some people worry about linking to websites like that because if the site stops doing business, your images disappear from your posts, but I've never heard of a problem with Photobucket.
It's best, though, to learn how to resize them with whatever photo software you have.
Nanci