By installing Exify's photo editing extension you can add two things to your photo: a watermark and textual metadata.
 

The product site has an in-depth tutorial with screenshots.
 

To install the Exify extension:
 

• First select an image in the Photos app and then tap "Edit".
• Next, locate the editing button (the circle with three dots inside) and tap it to show your extensions.
• Tap "More" to enable and arrange the Exify extension.
• To Finish, tap "Done", then "Cancel".

 

There are several options for the watermark that let you tailor it to your needs. The text you enter is recorded so it's quick and easy to add the same watermark every time. When you tap Done, the image is updated. If you want to remove the watermark, use the Revert feature in the Photos app.
 

You can also add metadata for copyright, image description, and creator. This data doesn't affect the visual appearance of the photo but can be very helpful in publishing workflows (Exify sets both TIFF and IPTC values.)
 

When an image contains location metadata, the editor can be used to remove the information for privacy.