How to add map icons

I have so many maps and many without icons that it is hard to remember what some of the maps actually are. I would like to create and add icons for maps that lack them. I have looked in the directory to find where they go but have failed. What file type are they and where do they go in the directory? Are they .jpg or even .vmt? I am stumped. I appreciate all the people who make good maps, I just wish they would take a little more time and add an icon for their map that would display in the GMod map area.