No, I mean say I want to use my own images of potion bottles instead of hearts. How do I do that?
Nope, that does nothing.
But I figured out - you need to create duplicate the variant, and edit that. Even then, you need to supply the image the right size, because the units (filled and unfilled) appear to be part of 2 separate tiled sprites. And you can’t resize the images that make up a tiled sprite.