Since the answer didn't explain why they occur and didn't really touch on why it matters, I felt like adding a bit more information.

BTW, when someone asks a question like this about artifacts, you should point them to section 2 of the Image Quality Evaluation page. That explains it fairly well and has some good examples. It helps a lot to be able to compare multiple copies of the same image with different levels of artifacting.

To put it really simply though, artifacts are the result of a sacrifice in image quality for the sake of minimizing file size. PNG compression never sacrifices image quality. But JPEG compression essentially always does.

Though, if you use the highest quality JPEG settings, even the sharpest eyes shouldn't be able to discern the difference. (When zoomed in switching back and forth between the two, you might see a very slight difference, but not enough to tell which is the JPEG version).

This is a site for high quality wallpapers. Artifacts lower the image quality, so we don't allow images that have too many.