ChrisHurst

MyWebsiteAdvisor

This plugin allows you to watermark your previously uploaded images using a PNG image file with transparency as well as text signature. 
The user friendly settings page allows for control over the appearance of your watermark. 
You can set your watermarks to always be a specific percentage of the target image. 
This plugin will add the watermarks to ALL of the images in your WordPress uploads directory.
Please BACK UP all of your uploads via FTP before using this plugin!
<a href="http://mywebsiteadvisor.com/plugins/bulk-watermark/" rel="nofollow ugc">Upgrade to Bulk Watermark Ultra</a> for advanced 
watermark features including:
<ul>
<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=XkFXBjfzw2I&hd=1" rel="nofollow ugc">Bulk Watermark Plugin for WordPress Video Tutorial</a>:
<iframe loading="lazy" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/XkFXBjfzw2I?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&hd=1&wmode=transparent" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe>
Developer Website: http://MyWebsiteAdvisor.com/
Plugin Support: http://MyWebsiteAdvisor.com/support/
Plugin Page: http://mywebsiteadvisor.com/plugins/bulk-watermark/
Compare Watermark Plugins: http://MyWebsiteAdvisor.com/tools/wordpress-plugins/watermark-plugins-for-wordpress/
Video Tutorial: http://mywebsiteadvisor.com/learning/video-tutorials/bulk-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:

Bulk Watermark

Adds an image and/or text watermark to all uploaded images, using PNG images with transparency.

sverde1

Watermark RELOADED is a lightweight WordPress image watermark plugin that automatically adds a customizable text watermark to images when you upload them to your Media Library.
Use it to:
<ul>
<li>Protect your work from unauthorized reuse</li>
<li>Brand images with your site name, URL, or copyright notice</li>
<li>Keep control of your visual content with a simple, set-and-forget solution</li>
</ul>
You can:
<ul>
<li>Choose which image sizes to watermark: thumbnail, medium, large, full size</li>
<li>Customize text watermark font and color</li>
<li>Precisely control watermark position (corners, edges or centered) on the image</li>
<li>Preview your watermark before saving settings so you know exactly how it will look</li>
</ul>
The plugin adds:
<ul>
<li>A dedicated Watermark RELOADED settings page where you configure your watermark and see a live preview</li>
<li>A WordPress Dashboard widget that shows your current watermark settings and a preview of the watermark directly on the main Dashboard screen</li>
</ul>
<h3>Key Features</h3>
<ul>
<li>Automatic watermark on upload – newly uploaded images are watermarked immediately</li>
<li>Text watermarking – add copyright text, site name or any custom message</li>
<li>Custom fonts – upload your own TTF fonts to use for the watermark</li>
<li>Color picker integration – choose any text color for your watermark</li>
<li>Per-size control – enable watermarking only on selected image sizes</li>
<li>Live settings preview – see how your watermark text, color and position look in the plugin settings before they are applied to new uploads</li>
<li>Dashboard widget – quickly review enabled sizes, position, offset and a preview from the WordPress Dashboard</li>
<li>Simple, focused UI – no unnecessary settings, just what you need to protect your images</li>
</ul>
<h3>How it works</h3>
<ol>
<li>You configure your watermark text, font, color, and position in Settings → Watermark Reloaded.</li>
<li>You choose which image sizes should receive the watermark.</li>
<li>When you upload a new image through the Media Library or post editor, Watermark RELOADED automatically applies the text watermark to the selected sizes.</li>
</ol>
Existing images in your Media Library are not modified – only new uploads are processed.
<h3>Requirements</h3>
To generate watermarks, your server must support:
<ul>
<li>PHP 5.3+</li>
<li>GD extension for PHP</li>
<li>FreeType Library (for rendering text)</li>
</ul>
If any of these are missing, the plugin will show an error message so you can contact your hosting provider.
<h3>Planned improvements</h3>
The following ideas are on the to-do list and may be implemented in future updates:
<ul>
<li>Additional bug fixes and compatibility adjustments</li>
<li>Smarter handling of small images where the watermark might overflow</li>
<li>Support for watermarking additional custom image sizes added by other plugins/themes</li>
<li>Image-size aware watermarks and more flexible positioning options</li>
</ul>

Watermark RELOADED

Automatically add customizable text watermarks to new images on upload to protect your WordPress media library.

Bulk Watermark vs Watermark RELOADED