if the pics width is larger than (in pixels) 2592 or height larger than 1944, then the forum says, no thanks, that's too big for me to waste bandwidth, and time resizing.
You might try a hosting website, I use Photobucket, you can easily upload your pictures to that then use their information to post your pictures on any forum.
Go here:http://photobucket.com/
It's free and quite easy to use...