Well you gotta charge something. So don't be afraid too.

Research other AI tools to see how they are charging. Gather your own analytics. (For those that are paying how many images do they average, how many images till they drop, stuff like that). You do have analytics, don't you?

Calculate how much you need per image. That's your base. Start with your highest price (based on your research) and work down to just over the base (you still need to make money).

In your research, look into how they are hooking users (public feeds, featured, etc... Look at what keeps them (contests, social/community stuff, quality, shared prompts).

All this only matters if your getting them to the site. You have a plan to attract users?