Answers:
It's call a watermark and it's what they use to identify their photos and prevent unauthorized use of them.
Let's say-so a company puts photos up on some site and offer to provide copies, and someone else downloads them and uses them for their own purposes. If the photos don't enjoy some manner of watermark (like the "DEMO" on the photos), nobody would buy the photos. They would of late copy them sour the website.
Also, if the photo shows up somewhere else and the "DEMO" is still perceptible, the being who originally posted the photos know that these other culture stole his photos.
They don't want you to copy the representation. The watermark is to ensure any integrity of their business (so you enjoy to buy it) or that you don't pickpocket it and use it short their authority.