If I remember someone asked about this in the forum so thought I would see what I could come up with using just some css. So here is the code I used and a couple of examples of how it looks. In the code you will see this \00A9 which gives us the copywright symbol and after that is TEST CONTENT which is the text to display so you would alter that bit. Font size is the size of the text and color is the color of the text.
The code goes in your design studio custom css
Example of list page; note it adds it to the add content photo bundles as well
Here is the CSS I used
.photoListPage .photoListPage-entry .matrix-media-4-3:before,
content: "\00A9 TEST CONTENT";