Stomach is generally understood to refer to both the organ and the exposed abdomen. "on front" sounds like something ESL. We could reverse the alias from prone if we wanted but I think it is unnecessary.
In what English-speaking region is it common to refer to the front of the body as simply the "front"? In my experience it's normal to say "chest", "stomach", "belly", etc., whereas the back is simply the "back", or if you want to be specific, upper or lower back. People suffer from back pain, not front pain. People get back tattoos, not front tattoos.
Changing the tag to "lying on front" reads weirdly and feels like forcing the tags into an arbitrary theme of opposites.
The tags on back and on stomach are for lying on your back/lying on your front so that is what's relevant. The words that are used in any other context are not relevant.
Especially as both of those examples given are due to anatomy reasons, in that the chest and the abdomen (often referred to as the "stomach") are quite distinct, for tattoos or for sources of pain. For when you're lying on both of them there is no such distinction (plus of course there are images in on stomach where only the chest part is actually being laid upon).
Incidentally, when someone has a tattoo covering the whole of the front of their body it's called a full front tattoo. Not a stomach tattoo.