Like they said, the short of it is that you need to post your pictures into one of the many photo-hosting sites. ImageShack.com seems to be the free service that is most geared toward hosting pics for forums. I like that it will auto-size the picture for its intented use.

I actually store most of my personal pics of vacations and family activities on Smugmug.com, but I only share that address with friends and family.

Once the picture is out there on the web somewhere, find out what its URL (web address) is, and then just enter/paste the URL for that picture into your forum message using the image button.