You'd need to find a host where you can upload your images. Most likely you'll want a free host which will allow off-site linking. In that case, I'd suggest:
http://www.photobucket.com Once your images are uploaded to PhotoBucket, just drop the URLs into some image tags (or BBCode) on the forums to show them. You can check the image tag use at:
http://www.oddworldforums.net/misc.php?do=bbcode
I'd promised oddguy I'd do a thirty second Oddworld doodle, but I've continually forgotten. Not too mention I don't have my scanner handy at the moment. Bah! I'd forget anyway
Alcar...