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).

marekki

Marekkis Watermark-Plugin for WordPress can watermark your pictures an two different ways:
<ul>
<li>Insert your watermark while the picture is being uploaded.</li>
</ul>
After the activation every picture that you will upload with wordpress build-in media-uploader will be watermarked.
In the configuration-screen you can set up the position and the type of your watermark. It can be your logo (.png-file) with transparent background or a free text with color, font, size, shadow and transparency-level of your choice. See screenshots.
<ul>
<li>Insert your watermark on all chosen pictures from a directory on your web-server.</li>
</ul>
Marekkis Watermark makes possible to create a watermark on mediafiles that are already uploaded on your server. So you can mark all your old pictures with the new watermark.

Header Watermark vs Marekkis Watermark-Plugin