Thanks for the quick reply!
Not sure where vBulletin is actually checking the HTML premissions, i guess somewhere (showthread.php) starting from line 984 ... Maybe done by the vB_BbCodeParser class, i can only guess

So far i've no clue even where i could overwrite the HTML check directly in the vBulletin file.
Should there be this "#" in showthread.php line 1073 (3.8.0 US)?
I'm not giving up so fast but right now i just "convert" the HTML with the vBulletin HTML to BBCode function and don't care about the post format. Not a solution i really like and adding or changing the vBulletin BBCode to manage some basic picture positions works but i don't like it either.
Dose anyone has some deeper knowlage where the HTML check is "hidden"?