Watermarks on "Gallery plugin" images

I’m using current plugin, but can’t find a way how to add watermarks to images that are within plugin called “Gallery plugin”: https://www.ads-software.com/plugins/gallery-plugin/

On other images that are placed to Media library watermark appears, but how to add watermark to 3rd party Gallery plugin?

Thanks in advance!

https://www.ads-software.com/plugins/image-watermark/