Easy Digital Download Image Watermark extension allows you to easily add watermarks to your Easy Digital Downloads images. These watermarks can be an image copyright symbol, company logo or piece of branding text as a transparent PNG image.
Usage Instructions:

Create a transparent PNG watermark image (eg: something that says “Copyright Yourname” with your logo).
Go to the “Downloads” > “Settings” > “Extensions” tab in WordPress.
Upload the watermark image using the Choose Image button.
Select where you would like the watermark image to appear.
Create a New Download and upload an image. The watermark should appear on the image.

Note:If watermarks do not get applied after regenerating thumbnails, make the image paths relative e.g. /2012/06/watermark.png