Imager plugin for watermarks
Have a look a little further down in the Readme and you will see more information regarding Watermark.

From the Readme...

image: The image that is to be used as watermark. Just as the image parameter in the craft.imager.transformImage method, this can be an AssetFileModel, a string to a previously transformed Imager image, or a string to an external image.

{% set logo = craft.assets({ id: 11 }).first() %}
{% set watermarkedImage = craft.imager.transformImage(image, { 
    width: 600, 
    watermark: { image: logo, width: 80, height: 80, position: { right: 30, bottom: 30 }, opacity: 0.8, blendMode: 'multiply' }
}) %}


Also a link to a Watermark demo https://imager.vaersaagod.no/?img=6&amp;demo=watermarks