Touching the core files is the last thing I would want to do..
It seems as I dug deeper, plugin/theme functions are the way to go, although I’ve yet to figure out how to create static pages from there.
I like EMG’s first suggestion and I guess that should suffice, as long as I provide instructions to use that template and create the pictures page after theme activation.
But still, it’d be good to have it done automatically :/
Anyway thank you all for the suggestions!