I am a little confused by your question.
Assuming you are talking about posting a picture in the forum, if you uploaded it while building the message, (using
manage attachments) then the picture is there. If you meant you uploaded it to a hosting site, then you have to put the hosting site's URL in as an image html (see the icon on the msg edit page.) What URL goes in there is kind of up to the hosting service.
Hope this clears up the confusion on your end.