You can add a text watermark to your listing photos a couple different ways.
If you would prefer to watermark all images you import, or upload to your listing by default, you can turn on a setting to enable this. To do so, log in to your control panel, then do the following:
Click on My Listings.
Click on Featured Listing Settings.
Find the setting for: Watermark Listing Photos by Default.
Turn this setting ON.
By default, your watermark will display your domain name. If you would like to modify this text, you can do so in the custom watermark text field.
Click to Save Changes.
Another way to use watermarks on your image, is when you are uploading an image, a checkbox may be available to "Watermark Image". By checking this box, the images uploaded will be watermarked. This setting has no effect if the Default setting is enabled.