In 2008, on my own sites, and on popular pages, I spotted several Xrumer based attempts to signup and then post some BS text like "nice forum

" and the hotlink method outlined above. In my case, none of these BS comments went through because, in my cms, custom code for users is disabled and, in general, I first screen new users who join.
Indeed, making a G search, it turned out that several thousands pages were actually spammed this way, but most FAILed or got deleted shortly after (visible only in G cache). Thanks to that overdose, now we're more immunized... It was a bit like the mass campaigns some Russian guys made with bots to get link backs from PhpBB sites, signing up as users, some years ago (2007). That exploit is no longer possible (see BlueHAtSeo post about it). It is like "nice try spammer, thx for showing us... now we block this hole too."
So. Don't overestimate the number of places that will let you add custom images be it with BBS code or real html. Don't overestimate the number of "********" places of relevance out there (they will keep decreasing). And overall, don't assume that Xrumer is clever enough to know where the code can be posted and where not (that is supposed to be YOUR task).
I think for this method to work you have to it manually or hire someone who does it with attention. As for credit: the idea of hidden image / hidden hotlink is not totally new anyway, not even the 1x1 pixel image hosted outside. It is the same realm of ideas of 1x1 images used for visitor tracking and/or hidden insite links to bust spambots that follow them, via htaccess (one tutorial on cloth .net, years old).
Again (and it applies to most "tricks") don't waste time automating this before you master it manually; specifically, don't automate it at all since it is time-bound and mass-bound. Do it manually and selectively - it will take more to be noticed as problem (oh well it has already been) and it will be devaluated less quickly by engines.