You know the image you see when you're playing on B.Net before starting the game, I want to change it to a custom image but can't find where to do this...
Save your image as a 128*128 or 256*256 pixels targa (.tga) image.
Open your map, click on import manager, import the file, and rename it to war3mapPreview.tga. Make sure it is in no directory (not even War3MapImported\), the full path must be war3mapPreview.tga.