You might want to use an external editor to do additional work on a photo. If installed, GIMP and UFRaw are the default external editors for photo and RAW editing, respectively. If these programs are not installed, you must select your preferred editors by choosing Edit ▸ Preferences and selecting editors from drop-down menus of installed applications.
Once your editors have been set, select a photo and choose Photos ▸ Open With External Editor to open the photo with the external editor. Likewise, if the original photo is a RAW file, select Photos ▸ Open With RAW Editor to edit the RAW file directly with the set RAW editor.
When you complete your edits and save the file, Shotwell will detect the changes and update the photo. When external edits have been made, press and hold the Shift key in full-window view to show the original photo rather than the externally-edited one.
Reverting to original will erase any external edits.
If you externally edit a RAW photo and save the result to another image, such as a JPEG or PNG, Shotwell cannot automatically determine that the original RAW and the new image should be paired.
If you want to work within the resulting image within Shotwell, you'll need to import it yourself.