Create an account on a photo sharing site like "Photobucket", it's free.
Once you have done that, upload your photos to their site. Bring up the specific photo you want to post on the forum, you'll see a box on the right of the screen that list "Links to share this photo", you want o select the IMG or bottom one listed. Most of the time you and just click on it and it will say "copied", sometimes it doesn't work and you have to manually copy the code. Sample Code:
[ URL=http://s799.photobucket.com/user/CruznCorvette/media/son-someday-will-make-girl--large-msgCORVETTES_zps72602d57.jpg.html][IMG]http://i799.photobucket.com/albums/yy275/CruznCorvette/son-someday-will-make-girl--large-msgCORVETTES_zps72602d57.jpg[ /IMG][/URL ]
I added some spaces so you could see the code for the image shown below.
Past the your "copied" of "code" in the reply box on the forum and your image should show.