Categories: Other Plugins, 4.x (incomplete list), 5.x, Bundled plugin
As a general rule, b2evolution generates thumbnails of all the images that you attach to a post (more information about this subject can be found on this page: Caching and Cache Levels), and the Watermark Plugin lets you add watermarks to those thumbnails.
The goal of the plugin is to add watermarks to generated thumbnails that stay visible no matter the size. Very small thumbnails will not be watermarked. The originals are not watermarked because the thumbnail would be scaled down in the thumbnails (real site images) and it would look bad.