Add watermarks to images on store page as well as on summary block

I would like to add a watermarks to all the photos on a website (it is a photographer's website, and she wants all her images to have watermarks). The images are located on a store page as well as on a summary block on the homepage. I have tried many suggestions of css codes so far, and none has worked. There are already 2 css codes added to the website. I don't know if this makes it more difficult to add a new code? Any help would be truly appreciated.

Thanks a lot!

ps: the website is not published, so not sure about the url...