First of all, thanks Ivan! Happy user of Photopea.
Second, the trademark will help you against the masqueraders, those copying your tool and the Photopea brandmark. That will help with customers complaining about some other modified product. It will not help in case you find someone copying your codebase and putting it out in the open under a different name. For that, you'll need a copyright.
Doing both of these might be expensive but gives you complete legal standing. Companies will have no choice but to take down the copies.
Second, the trademark will help you against the masqueraders, those copying your tool and the Photopea brandmark. That will help with customers complaining about some other modified product. It will not help in case you find someone copying your codebase and putting it out in the open under a different name. For that, you'll need a copyright.
Doing both of these might be expensive but gives you complete legal standing. Companies will have no choice but to take down the copies.