You’re allowed to use copyrighted works for lots of reasons. EG satire parody, in which case you can legally publish it and make money.
The problem is that this precise situation is not legally clear. Are you using the service to make the image or is the service making the image on your request?
If the service is making the image and then sending it to you, then that may be a copyright violation.
If the user is making the image while using the service as a tool, it may still be a problem. Whether this turns into a copyright violation depends a lot on what the user/creator does with the image. If they misuse it, the service might be sued for contributory infringement.