Hehe funny you should mention it... (just discussing it elsewhere!)
A picpost is where single images(usually hotlinked from the submitters site) are submitted and displayed on the post site and linked to the submitting site, with/without a banner.
Like a few other formats ....they are under-utilised.
It's looking a bit dated now but one of the originals..
http://www.picpost.com/ is still up and functioning... (look for "Today's PicPost Listing")
DD