kdmurray

This plugin will generate an tag for a random image selected from a specified folder.
<h3>Setup Instructions</h3>
<ol>
<li>Extract randomImage.php into your wp-content/plugins folder (or a subfolder)</li>
<li>Activate the plugin in WordPress</li>
<li>On the options screen, select the “Random Image” menu</li>
<li>Fill in the values for your physical and http paths</li>
<li>Select scaling options (optional)</li>
<li>Add some code to whatever page you want to display the image. This is a great way to 
customize the header image for your wordpress installation. The plugin will look in 
the folder and randomly select a header image to display.</li>
</ol>
<h3>Sample Code</h3>
By placing code like the following in your header, it will create an image tag for you 
based on the images in the specified folder.
<?php 
 if (function_exists(‘generateRandomImgTag’)) 
 { 
 generateRandomImgTag(); 
 } 
 ?>
<!– 
The lines above are for the HTML documentation (online)
–>
This code will ensure that the function exists (ie the plugin is active and working) and 
call it if it’s working correctly. If the function is missing, or the plugin has been 
removed then the call will be ignored, and no errors will result.
<h3>Feedback</h3>
kdmurray.at.kdmurray.dot.net

Random Image Selector

This plugin will generate an  tag for a random image selected from a specified folder.

MyWebsiteAdvisor

ChrisHurst

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.

Random Image Selector vs Bulk Watermark