Issue with Gravatar is that you need to use the same email address so if you use different ones on different websites, it won't work (unless you can register multiple email addresses).
I suspect no-one is really that fussed about seeing a picture of the peeps on here (unless it is the blokes trying to check out the females (and then discovering they are actually blokes!)), but Gravatar has probably done a block change for any unregistered accounts (although I don't recall ever doing anything on Gravatar so it must be some sort of template change done on the actual profile page when you register).
Pentagon with a squint smile…not a bad guess, but it's of no interest to me…