Press F5 in-game when you want the screenshot to be taken. You can take as many as you want. The files show up in your Half-life folder (or cstrike folder for cs). They will be a bmp file and will have the name of your map with some numbers added at the end. for instance mymap0001.bmp. In order to place a screenshot into the map vault you have to convert it to jpg format. To do that, I use MS paint to open the bmp and just save it again as a jpg. Now let us admire your beautiful map.