Michael Zangl

This plugin allows you to add a watermark images on all images that were uploaded in your wordpress. The watermark is generated on-the-fly and cached for faster access.
To see the plugin in action, select “Add watermark to all images”, chose a watermark image and and submit the settings page. You should then see the watermark appearing on your site. You might need to reload the page (F5 in most browsers) to bypass the browser cache.
The plugin does not change your source images. To disable all watermarks, simply disable the plugin, and everything is back to normal.
If you encounter errors (especially syntax errors), check your PHP version. It should be at least 7.0. This plugin requires Apache and mod_rewrite.

Add Watermarks

Adds watermarks to selected images without changing the original image.

daxiawp

Automatically add a picture watermark, can select text or pictures are two types. 自动给图片添加水印，可选择文本或图片两种类型。 
详情：<a href="http://www.daxiawp.com/dx-watermark.html" rel="nofollow ugc">http://www.daxiawp.com/dx-watermark.html</a>
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DX-Watermark

The pictures automatically add watermark. 图片自动添加水印.

Add Watermarks vs DX-Watermark