You said you want a custom one; if you want a custom one, just do as I said: create an image in an image editing program, of size 256*256, paint it the way you want, save it as war3mapPreview.tga and import it via the Import Manager in the following path: "war3mapPreview.tga" (meaning delete the "war3MapImported\" prefix). If you want to return to the original minimap preview, go to Scenario -> Map Options and untick the "Hide minimap in the preview screens".