Usually something added to the image by the place that did the scan, like a web address, or "copyrighted material".

If you scan it yourself, you should be fine.

Some are nearly impossible to see nowadays, unless you know where to look and get the right angle on your monitor. Some of the dvdempire scans have black hidden in black areas. In fact, recently something that slips my mind now had an audit up with new scans that mentioned the old were watermarked. I never could find the WM on what was up until I took the picture and cranked up the contrast, then it showed up.