Legal ramifications of using free desktop wallpapers on websites?

Does anyone know for sure or know where I can go to read the rules are about downloading free wallpapers, then using them as images or backgrounds on websites? The download sites never seem to cite any policies regarding licenses or prohibitions for use. I’ve been googling for a half hour. Would appreciate some insight.

Simple:

If it’s not your original work, don’t use it.

If it’s not your original work and you don’t have explicit permission from the creator to use it in the way you want, don’t use it.

If you hire anyone, make them adhere to these two rules. Produce only original work for your site.