I know how to get a picture for when you loading up the game right before you play but how do you put a picture up for when people are DLing the map and when you select the map to host it.
You have to create a 128x128 or 256x256 sized, 32-bit TGA [Truevision Targa] image and import it into your map [via Import Manager]. Then, give it path "war3mapPreview.tga".