agnu

Header Watermark provides you with extra flexibility and control over the header image(s) for your WordPress site:
<ul>
<li>Use any image from your Media Library as a watermark for your header image(s)</li>
<li>Control the placement and opacity of the watermark</li>
<li>Select any suitable image from your Media Library as your header image(s)</li>
<li>Control which images are used for random header images</li>
</ul>
<h3>Requirements</h3>
Header Watermark requires the following to work:
<ul>
<li>GD Library PHP extension – version 2.0.28 or higher</li>
<li>The WordPress theme must use the get_header_image() function to generate the header image and must also define the HEADER_IMAGE_WIDTH and HEADER_IMAGE_HEIGHT constants</li>
</ul>
<h3>Themes</h3>
This plugin works with the Twenty Ten theme with no further configuration.
If you use this plugin with the Twenty Eleven theme, you may see a notice at the top of your website/WordPress admin pages like this:
<blockquote>
Notice: Constant HEADER_IMAGE already defined in /var/www/yoursite/wordpress/wp-content/themes/twentyeleven/functions.php on line 118
</blockquote>
You may also find that you get occasional errors on some WordPress admin pages, for example if you activate a different theme.
To prevent the notice and errors, find and change the following line in the functions.php file of the Twenty Eleven theme:
Change this: 
 define( ‘HEADER_IMAGE’, ” ); 
to this: 
 if ( ! defined( ‘HEADER_IMAGE’ ) ) { 
 define( ‘HEADER_IMAGE’, ” ); 
 }
Any other themes that define the HEADER_IMAGE constant may have the same problems and need to be altered in the same way for the plugin to work.

Header Watermark

Add a watermark image to your WordPress header image and use suitable images from your media library as the header image(s).

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" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/pg3WvPBliM4?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/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.

Header Watermark vs Signature Watermark