Inspired from something 13coinmike posted previously. It's a lot easier than it looks once you do it a few times.
Open coin picture in MS Paint and extend right border to at least twice coin diameter:
Select oval tool and draw oval from top to bottom:
Extend left/right of the oval to coin edge:
Then drag the circle to the right:
Select the fill tool (with black foreground) and fill outside the circle:
Select the select tool and turn on the transparent selection:
Draw rectangle around the white circle:
Select the rectangle and drag it left until the coin is shown with black surrounding it:
Select the fill tool with white foreground and fill the black:
Crop to edges of coin:
