what does it mean when someone has DEMO on their pictures basically all over the image?

Answer 1

It's called a watermark and it's what they use to identify their photos and prevent unauthorized use of them.

Let's say a company puts photos up on some site and offers to sell copies, and someone else downloads them and uses them for their own purposes. If the photos don't have some kind of watermark (like the "DEMO" on the photos), nobody would buy the photos. They would just copy them off the website.

Also, if the photo shows up somewhere else and the "DEMO" is still visible, the person who originally posted the photos knows that these other people stole his photos.

Answer 2

They don't want you to copy the image. The watermark is to ensure either integrity of their business (so you have to buy it) or that you don't thief it and use it without their permission.