I just came to post up about this as well. NONE of my forum members know html, and they ALL want images in their signatures. 

What about a popup editor, where they can design their code, then copy and paste it into the signature field, would that be a workaround for whatever conflicts you guys are experiencing in the profile coding? (sorry, I don't know all the terminology) Or an iframe of an editor where they can design, copy and paste...

all part of the problem getting tinymce active in...  and significant redesign of the profile panels which are currently heavy ajax...

but we do understand the desire and have a ticket as mentioned and hope to tackle it soon...  probably post 5.5.2 due out end of month...  too many business trips and vacations for team this month to tackle this big project...