How to add a signature to your photos

Check out the Mac App Store for applications that can watermark images. Look at the higher star-rated products first. Search for image watermark in the Mac App Store search field. No assurance that any of these will work within iPhoto or Photos.

Using iPhoto (and not Photos?) then use the free plug-in iBorderfx

http://www.iborderfx.com

Signature? DO you mean a watermark on the image? How are your photos managed?