For posting to a forum thread, there is a 200 kb limit. My personal recommendation would be to keep 'em 640x480 or less, but if they're under 200 kb, any size you want will work. For posting to the gallery, the limits are 1 mb and no larger than 4000 pixels either direction. My personal recommendation for gallery posting is to try to keep 'em also under 200 kb whenever possible, and that's just because a whole lot of people still use dial-up and are more likely to view an image with lower bytes. Just don't confuse my suggestions with the actual limits. :)