I know we can currently 'add others' to an activity, and I also know that we 'group' with others - but it would be great if we could tag others in photos we upload. There has been at least one occasion (while on a run), where I ran into somebody I know - we snapped a selfie (if there is more than one person in a photo, is it still a 'selfie'?), then continued on our way... we didn't group together in the activity, but would be great to be able to tag in the photo as a way to link our runs.
Perhaps after photo upload, you could click photo and 'tag others'. I think from a 'privacy' standpoint, we could have a setting in our account to 'allow others to tag you', so that you can prevent unwanted tagging (if a Strava Athlete then did try to tag you, they would get a message saying "This Athlete has elected not to allow tagging" or something similar).