I managed to do this
But that also means you'll be stuck with this
What you need is to scale your image to fit as a minimap and make it a tga file. Then you must name the file MinimapB.tga
Just import the image to your map and when it is imported you must rename it Minimap.tga. Then save. The Minimap.tga file will disappear from the imported file list, but it will show up when you load the map.
(your image can not be imported if it's called Minimap.tga, the editor won't allow it. So you must first name it something else, like MinimapB.tga, then rename it while in the file import manager)
I'll upload my test map and the tga file I used so you can tinker with them if you need to.