Click on the picture in your album, which will take you to a page and make it bigger. Then, mouse over the big picture. A little bar will pop up over it. Mouse over "share", and then click the "more options" at the bottom of the share list. This will open up a new window. Click "get link code", the third choice at the top of the grey box in the page. Go down to "IMG for bulletin boards and forums". There will be links for your picture there. Copy the one you like, and then paste it onto the forum.
Hah, hope this wasn't too confusing! XD