Any update on this issue please? Thank you.
This should get worked on, but because bbPress does not use TinyMCE by default, and because image uploads are not enabled by default, it is likely to be a low priority fix.
Created https://bbpress.trac.wordpress.org/ticket/3304.
I should clarify that the image has to be already online. In TinyMCE you just provide a link. Just like with BBCode.