A common way to prevent art theft is to add a watermark to display images. This can be a simple grid of text that says "display" or your user name at low opacity. For example:
You can make your watermark as simple or complex as you want. You can also make the watermark more visible. So long as the watermark would be obvious if someone tried using the art from the display image, you're good.
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
