Custom minimap previews for custom scenarios is a big fashion. Some people might actually leave the game you've hosted because they think that the map sucks when it doesn't have a minimap preview. So here's a guide on how to make your own.
Open your favorite image editor and make yourself a good picture. You have to keep in mind that the image should have the dimensions of either 256x256 or 128x128.
Save it as .jpg or .tga if you can. If not, go get the tool Warcraft 3 Image Extractor II, open up your image and save it as .blp or .tga.
Open up World Editor and then open the Import Manager and import the image.
Double-Click on the image and tick "Use Custom Path" and set the path to: war3mapPreview.tga or war3mapPreview.blp to have the image being the minimap preview.
You can also save it as war3mapMap.tga or war3mapMap.blp to make it being the in-game minimap.
As far as I understand, the problem is in the .tga file, but I don't know much about that. So use the Warcraft III Viewer to convert it to .blp. If it asks you to add an alpha channel, say Yes. Then import the .blp with the path War3MapPreview.blp.