Here's what I did...
I have a bunch of images in my gravatar account, but stackoverflow always seemed to display the same one no matter which was picked on gravatar.com. One day my icon spontaneously changed to a different image, though not the one which I had selected.
I decided I'd better fix it, so I clicked 'Change picture' on my SO profile and selected the image I wanted, noting SO's warning about it taking time to change.
A weekend passed, and this morning my SO icon is back to the original (wrong) icon.
Extra info: I've had support queries before regarding my accounts needing to be manually tied together on the different sites because it had become rather muddled. I wonder if this has somehow got something to do with my strange gravatar behaviour.