There are a couple of people working to make it easier. But in the simplest sense...
If you load an image of Madonna into a paint program, remove all COMPLETELY white pixels (255, 255, 255) which is usually done just by makng them a light grey. Then put the tiniest possible white dot in each location that would be a "flag" when you played it. Then you can load that image into the Map Editor in the Tools part of Dominions and have it generate a .map file for it.
The results would be bad.
But yes its possible. Usually its better to choose a decent image (like a sattelite photograph of Europe?), add borders, then the dots, and reasonable neighbor connections.