Simply save this image as "rank_graphic" inside the corresponding rank folder.
It needs to specifically be "rank_graphic.png". (Eg. "rank_bitmap.bmp", which Paint would default to, won't work.)
I'm creating those with an ancient version of Adobe Photoshop (7.0). That way I find it easer to layer things on top of each other, add drop-shadows or outlines, make layers invisible again etc. Don't know to what extent such things can be done with a modern version of Paint.
As far as I know, even the newest version of Paint doesn't have layer support. However,
Paint.NET is free, very simple (compared to Photoshop or GIMP), and supports layers. It's what I use for all my Lemmings graphics work.