I'd advise you not use the example of removing a watermark. It makes it seem like you're advertising the app to be used for circumventing photographers' copyright/attribution.
You should really make people aware that they are uploading their photos to your server, and provide some sort of privacy policy. This is a total dealbreaker for me.
I was really impressed by remove.bg but hate uploading my pictures to who-knows-where. I looked around GitHub and there are several projects to remove backgrounds from photos but hard to judge the quality without trying them. Most were some python blob with murky or no instructions on setting up. But give it a few more years and I’m sure there will be a gimp plug-in.