Oh, that's a good concern! For that, I've prepared a whole list of public domain & creative common resources on the jam page that are free to use and should be 100% fine. Other popular stock photo websites also offer free limited download options. Ultimately, it's up to the jam participants to respect the copyright and follow asset licensing terms, but it's no different to using any other 3rd party resources like 3d models, sprites, music and sounds, which is very common when making an indie game.