With or without a watermark, if someone were to download a display image and use it in their own project it would look blurry and unpolished.
Also, projects with stolen art are at risk of having their project taken down or, at worst, hearing from my lawyer.
At the end of the day though, watermarks are just a simple way to ward off lazy thieves. If someone really want to steal art, they will find a way to do so. There's no absolute fool proof way to prevent it