- Create a 32 bit Targa image (*.tga) using following resolutions: 128x128 or 256x256. - Run World Editor and then open Import Manager. - Click "Import file" button. - Point to your TGA minimap. - When image appeared on the list, double-click it. - Click "Custom path" checkbox. - Remove path string and type "war3mapPreview.tga".
For Loading screen search tutorial. If you want more info, search it in (4)Monolith.w3x in "Warcraft III\Maps\FrozenThrone\Scenario\" directory.