I want to make, and add more matcaps. How do I add them?

Hello. I’m getting tired of the default matcaps in Blender, so I want to make some new ones. I figure I will just render a sphere the way I want, and that will be the matcap. My real question is, once I get my image, where do I put it so that when I open the matcaps tab, I see my new ones. Do I have to import them the way I would a brush? How do I get them?

That works great! Thanks!