The watermarking programs that I tried had similar systems. The user can upload one photo or a batch of photos, then select an image to apply to the photos. Text can also be added instead of an image. The user can also decide where on the image to place the watermark, and set the transparency level of the watermark. Once the watermark is chosen, it can be applied to a batch of photos with just one click. This really saves time when working with a large group of photos!
Numerous watermarking programs are available online. I like the interface of this particular one and found it fairly easy to use.