However if you read between the lines your manager did give you an out that being don’t let the “extra” work interfere with your “assigned” duties, therefore going forward I would suggest no more editing on your own time or with your own PC, work photos will be edited at work on a work PC on company time, if getting those photos processed becomes excessively delayed maintain the upper hand and offer to train a co-worker in the process to assist you with editing them or have them temporally shift some of your “assigned” work to someone else, then once the work is shifted away leave it there and gain some FO time.
I like this approach. Work on the images when you have "extra" work time available to do so. If there's not enough (or any) "extra" work time to get the images processed, they wait. You could even politely update them by saying something along the lines of "Sorry its taking so long to get the photos ready, I'm currently swamped with my normal job work. I'll get to them as soon as I have time."
I do free photos for my employer, but its in a very limited circumstance. We have an employee giving and volunteer program that I fully support. At the various volunteer programs and volunteer days they run, they ask a handful of us hobbyist photographers if we'd like to take photos of the event. I'm happy to do these photos for them because they're used primarily to promote the volunteering program internally, and sometimes outside the company.
However for any professional photography work - corporate headshots, PR events, etc., they hire professional photographers.
And some 40 hours per week jobs do still exist - I've got one of them. There are occasional times when things get too busy, but those are not the norm.