The General Settings provides options on how to enable and apply the watermark. There are two ways to apply the watermark: Automatic and Manual.
Automatic applies the watermark when a new image is uploaded, instantaneously. This option is useful for applying the watermark on many images at once.
Manual requires the user to select images in the Media Library to apply the watermark on. This option is useful for existing images.
Enable watermark for option has several sub-options. At least one of these options needs to be enabled for the watermark to apply.
full: Selecting this option will apply the watermark on the original image as uploaded to the site. This is not recemmended! You may need these to remove watermark from images.
large: Selecting this option will apply the watermark on large image size.
medium: Selecting this option will apply the watermark on medium image size.
thumbnail: Selecting this option will apply the watermark on thumbnail image size.
Custom Image Sizes: On some themes custom image sizes will be specified. These custom image sizes are theme related and are not related to the plugin. In this example, ambition-featured large, ambition-icon, and ambition-recent-work are all custom image sizes associated with the theme. Custom image sizes will be listed along with default images sizes available in all WordPress installs.
Everywhere: This option allows the watermark to be applied globally, across all posts and pages.
On selected post types only: This option allows the watermark to be applied on selected post types, like posts, pages and custom post types
Allows the watermark to be applied to images uploaded to WordPress media library from the frontend of the site. This requires a custom script or a frontend uploading plugin, for example Frontend Uploader.
Scroll down and find the section labeled, Watermark Image. Then upload an image. To remove an image click Remove Image button.