Well, go to your level. In the console, type CHASECAM 0 and SHOWHUD 0. Go to whichever spot in your level that you'd like the picture to be at, and press F8 to take a screenshot.
Go into Paint or whichever image editing program you use, and open up the screenshot. Either resize or crop it down to 160x100, then save it as a .bmp file.
If you don't have XWE, get it. Open up your wadfile in XWE, and import the level image. Rename it to MAPxxP. (For example, if your map is in MAPA1, put MAPA1P, and if it's MAP01, put MAP01P.) It should then work.