Rather than copy/pasting the link info to the post window directly you need to click the "add picture" icon - it is the little box that looks like a sun and mountains up htere in the boxes at the top of the post window (Size, A, B I U link unlink and then Insert Image - next to the smiley face) - then you paste the link (for Photobucket you copy the "direct" code next to the image) into the popup window in the box for "An Image URL" and hit "Use URL" - a new box will come up, select the size you want it and hit "Submit"
