A small corner mark is fine for photos you are happy to share. When you are sending work that someone could use without paying, such as a photographer's proof gallery, a designer's draft or a product mock-up, a mark repeated over the entire image is far more effective. There is no clean area to crop to, and removing dozens of overlapping marks by hand is tedious.
This page opens the Watermark tool with tiling already selected. All the same options are available if you want to switch to a single mark.