cedar: I've added it to your board's css Reph. You shouldn't be seeing anymore of the default Yuku avatars now - only the ones that people have personally selected.

(CSS is the board's style sheet. You'll find it in 'admin > design box > customized skins'. You're using the Yuku Professional skin, and if you click on 'edit html' you'll see the css in the third box on that page. The first two are for the html header and html footer.)
We changed to a different skin for the Off Topic forum, the only one that allows avatars, and the blue dummy avatars are back. I followed your directions and looked at the contents of the third box, but I don't know what to change them to. Delete everything in that box, or what?