actually, if you can find an even higher quality overview picture of de_dust2, simply put it as env_image in the map, then put a semi-transparent env_sprite the same size in the same position as the env_image to work with temporarily. then, create a simple tileset of blank tiles (transparent even will work) and just have enough to have one be wall block, one be obstacle block, another be floor-dirt, and a floor-tile. then, place those tiles accordingly on the map to match up with the overview picture. make sure you use blocks (no shadow) so that random shadows don't appear on top of the image. Once you're done, simply delete the env_sprite, and you're all set!
This is kind of a cheap way to do it, but it's just an easy suggestion so that you can have the map literally look EXACTLY like de_dust2 from CSS.