It's easy!

You need to find the pictures URL (this is the specific website address of the photo).

So, if you see a photo on a website that you like, right-click on the photo and, depending on your browser select "copy image url" or click "properties" and then copy the URL, the "address" of the photo.

For example on this website

If you right-click on the first photo and bring up it's properties you'll see it's URL (address) is

Copy that. Click the little image button above

Paste it in..and you should get...