First thing to do is to up-load them onto photobucket, then copy the image code from the full sized pic into a reply on the forum.
To do this there is a row of icons at the top of the reply window, click on the fourth from the right and [ IMG][/IMG] will appear in the message, copy the image code between the two middle ] [ and with any luck the picture will appear when you post the reply. You can always check the preview to see if it's worked before posting.