How do I make a preview Icon for my map?

I made a map about a year ago and never released it to the public. I want to upload it to garrysmod.org or garrysmod’s toybox even (If I can ever figure out how to do it); however I don’t want it to show up in the map list with a blank icon. I’ve been searching for tutorials on how to do this, and I can’t seem to find any. Does anyone here know how I can go about doing this?

This is the map btw: http://www.youtube.com/watch?v=nTsba9eVPRk