How do you add a Map picture in COD menu -1 reply

Hi all you mapsters, I've noticed in a few maps I've downloaded that you don't get a preview map when you go through the map menu in the game. I tend to forget what some of them look like and would like to add a suitable screen shot for my own benefit. Can anyone point me in the direction of which file type to look for in the PK3. I,ve looked in the PK3 files for maps that do have a picture using Pakscape and I can't find anything that resembles a picture file type. Thanks Private Frankster