mitcho (Michael Yoshitaka Erlewine)

This plugin adds a big red “preview” watermark to post previews. Never again be confused whether you’re looking at a preview or a published post!
Note: This plugin may not work well if your theme involves a background image on your <code>body</code>.
This plugin is a component of the <a href="http://ecs.mit.edu" rel="nofollow ugc">MIT Educational Collaboration Space</a> project.

Distinct Preview

This plugin adds a big red "preview" watermark to post previews. Never again be confused whether you're looking at a preview or a publi …

MyWebsiteAdvisor

ChrisHurst

This plugin allows you to Automatically add a watermark to all images as they are uploaded to the WordPress Media Library. 
The plugin uses PNG watermark images with transparency for precise control over the appearance of the watermarks. 
This plugin also supports simple text watermarks with adjustable color, size and transparency. 
The plugin can be configured to apply ‘text and image’, ‘text only’ or ‘image only’ watermarks. 
The user friendly settings page allows for control over the appearance of your watermark. 
The watermark preview feature allows for easy testing of the plugin settings. 
The watermark size is controlled as a percentage of the target image, 50% means the watermark will be half the width of the target image.
<a href="http://MyWebsiteAdvisor.com/plugins/signature-watermark/" rel="nofollow ugc">Upgrade to Signature Watermark Ultra</a> for advanced 
watermark features including:
<ul>
<li>Manually Apply Watermarks to Images Previously Uploaded</li>
<li>Fully Adjustable Text and Image Watermark Positions</li>
<li>Adjustable JPEG Image Output Quality</li>
<li>Highest Quality Watermarks using Image Re-sampling rather than Re-sizing</li>
<li>Lifetime Priority Support and Update License</li>
</ul>
Check out the <a href="http://www.youtube.com/watch?v=pg3WvPBliM4&hd=1" rel="nofollow ugc">Signature Watermark Plugin for WordPress Video Tutorial</a>:
<iframe loading="lazy" title="Signature Watermark for WordPress Video Tutorial" width="750" height="422" src="https://www.youtube.com/embed/pg3WvPBliM4?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
Developer Website: http://MyWebsiteAdvisor.com/
Plugin Support: http://MyWebsiteAdvisor.com/support/
Plugin Page: http://MyWebsiteAdvisor.com/plugins/signature-watermark/
Compare Watermark Plugins: http://MyWebsiteAdvisor.com/tools/wordpress-plugins/watermark-plugins-for-wordpress/
Video Tutorial: http://mywebsiteadvisor.com/learning/video-tutorials/signature-watermark-tutorial/
Requirements:
<ul>
<li>PHP v5.0+</li>
<li>WordPress v3.3+</li>
<li>GD extension for PHP</li>
<li>FreeType extension for PHP</li>
</ul>
To-do:

Signature Watermark

Automatically watermark images as they are uploaded to the WordPress Media Library using Both Images and Text.

Distinct Preview vs Signature Watermark