@Rainbow Dash - as Vesper says, though Elsa was still a child, her maxed mental skill allowed her to gain points in Video Gaming. I knew beforehand that kids could learn some adult skills, but went googling for a full list; that was also when I learnt about them being able to learn Vampire Lore, courtesy
this list:
Maxed Creativity unlocks: Media Production, Photography, Piano, Pipe Organ, Singing, Violin.
Maxed Mental unlocks: Fishing, Logic, Video Gaming.
Maxed Social unlocks: Charisma, Mischief. (Not Comedy for some reason.)
Maxed Motor unlocks nothing.
Available from the start: Bowling, Pet Training, Vampire Lore.
Apparently I miscategorised Media Production when I replicated this for my spreadsheet, which might explain the trouble I had in testing! Whoops! One other thing to note - I was actually aware that Video Gaming was accessible to kids already, and moreover, that it can be unlocked by maxing either Mental
or Motor, because I remember quite clearly a bug where any points gained in Video Gaming after maxing one of those two were lost the moment the other was maxed! I assume it's long since been fixed, but was still pretty worried about Elsa in this challenge regardless. She was gaining Motor skill alongside Video Gaming and almost as quickly, but fortunately having the club perk skill boost for Video Gaming and the Focused club vibe (Motor being an Energized skill) meant she was able to reach level 10 before maxing Motor, though I think she did make it to level 9 there!
As for the photos, taking phone selfies with sims is the way I went, hence the shot of Skylar and Dennis Kim, but as has been said, simply taking a photo of another sim works too. Oh, and with regards to your original strategy post - you don't have to kick yourself for forgetting gym trainers, because I forgot about handymen!