I'd say that's exactly what he is referring too Zubaz.
This is the images page from within my profile on GC2, the page is identical in layout to the one at SD Forums, where i assume you are seeing this JCD.
As for what they do, i've never seen them in use on forums other then the Avatar of course. I have always assumed it was a way to show off a pic of yourself *Personal Image* and the actual desktop *Desktop Image* pic you are using as your wallpaper, note it allows a much higher res, suggested this might be the case.
SD's roots were and still are in desktop customization, so "showing off" your current desktop pic is something often done.
