How to add metadata to images using Shotwell
Solution 1:
Such a feature is only available in Shotwell itself. 'Shotwell photo viewer' lacks all advanced features and is, as its name suggests, only ment for viewing images.
How To: Open Shotwell, and if you haven't done this yet, import your photo collection by setting its location in the preferences window.
Now you can edit the title, comments and tags of any photo. (by right clicking it, or using the options under 'Photo' in the menubar)
Shotwell can save this meta-data in the image files them-self, or in a separate database. This can be set in the preferences window.
Solution 2:
If you open Shotwell, double click on the image to preview it, then click on Photo item on the main menu you get the options to Set rating, Edit Title, Edit Comment and Adjust Date and Time