Since you don’t ask for any “easy” way, the best way would be to arrange the pictures (all set as 3D layers) in their final position, at the point on the timeline where you want them settled there, then back up to the point where you want them to start coming in, and move them to wherever you want them to start. Make sure you turn on the Position keyframe at the final landing point.
As far as arranging them, if you want them in a symmetrical grid arrangement, there are some scripts which could speed that process up:
https://aescripts.com/layers2grid/
https://motionboutique.com/collage-script-328/
Collage script allows you to take non-uniform layers, and create interesting arrangements of them. It’s also free.
Joe Bourke
Owner/Creative Director
Bourke Media
http://www.bourkemedia.com