Import a custom 256x256 or 128x128 .tga image, and rename it in the import manager to "war3mapPreview.tga".
Make sure "Hide minimap in preview screens" is unticked.
As a side note, this solution could have been found EASILY with a quick visual search at the tutorials, or even a google search.
In future you should try search a little before asking questions that might have been answered before; usually you'll get your answer faster than if you had make a thread!