Bossk in Return of the Jedi?

That's a great website

 

Without doing a frame-by-frame on my DVD, I don't think Bossk is apparent in ROTJ. If he's there in the film, he's probably in the background with blurry Yuzzums & 'Mara Jade'.
The picture of him on the sail barge noted above looks like a publicity shot, it's possible he was in the background of the deleted scene where C-3PO has to break up a fight between Ree-Yees & someone else (haven't seen any stills of this actual scene, either).