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.

Marekkis Watermark-Plugin

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 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. 
Watermarks are now removable with the new backup system, any images watermarked while the new backup system is enabled are able to be restored to the original image.
<a href="http://MyWebsiteAdvisor.com/plugins/transparent-image-watermark/" rel="nofollow ugc">Upgrade to Transparent Watermark Ultra</a> for advanced 
watermark features including:
<ul>
<li>Manually Apply Watermarks to Images Previously Uploaded</li>
<li>Fully Adjustable Image Watermark Position</li>
<li>Fully Adjustable Text Watermark Position</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=fEhZK1U8W94" rel="nofollow ugc">Transparent Image Watermark for WordPress video</a>:
<iframe loading="lazy" title="Transparent Watermark Free Plugin for WordPress by MyWebsiteAdvisor" width="750" height="563" src="https://www.youtube.com/embed/fEhZK1U8W94?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/transparent-image-watermark/
Compare Watermark Plugins: http://MyWebsiteAdvisor.com/tools/wordpress-plugins/watermark-plugins-for-wordpress/
Video Tutorial: http://mywebsiteadvisor.com/learning/video-tutorials/transparent-image-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:

Transparent Image Watermark

Automatically watermark images as they are uploaded to the WordPress Media Library.

Marekkis Watermark-Plugin vs Transparent Image Watermark