Don’t know how you’re rendering still images, but if you’re using Movie Render Queue,
you can do this using Widget Blueprints.
Create a Widget Blueprint, called something like WBP_Watermark.
Customize it to your liking (add any text, images extra that should show up as watermark.)
in Movie Render Queue, add UI renderer.
In your Level Sequence, add an Event Track. And add a Trigger keyframe and bind it to a new event.
From that event call Create Widget and select the WBP_Watermark widget.
From the return value of the create widget node, call Add to Viewport.
Result :
