I don't think this can be done, not from what I see using the TinyMCE editor. Tried to insert a full image size attachment and position it right so text could wrap around it. Not an option I'm guessing?
I would love to see this in the core system.. But a plugin would be nice to.. Im kind of OCD and hate that i cant wrap text around images.. It just looks better and gives the forum a professional article feel.
Mainly spotted it because I uploaded a movie cover as an attachment, only about 100x30px in size. Wanted to float full size image top-right so text wrapped downwards around it. Just more for better presentation.
Doing it with CSS in an HTML webpage is dead easy.
Just checked the TinyMCE demo and looks to me it can do it. Take a look at the image positioned top-right, seems that feature option is just not used here which should be in there for activating.