The quick tip for making an animated photo mosaic in After Effects is to generate the mosaic externally and animate with the built-in Card Dance plug-in, which can be a little tricky.
Using Ayato's method in video is Photo Mosaic Animation Tutorial by Kevin Adams, a clear & basic video explanation of using Card Dance for a mosaic in AE, but without the extras used by Ayato. Also available is Face Mosaic by Rob Hindley, a free After Effects Project on Adobe Exchange that Todd Kopriva dubbed a “clever use of Card Dance and Fractal noise.”
A Total Training video from Brian Maffitt (he created the effect) on Using gradients to control the Card Dance plugin's behavior, might be useful. Another tutorial could also be useful, 21. CARD DANCE EFFECT by MaxAfter. And, the After Effects: Effects A-Z tip series from Motionworks hosted a James Zwadlo demo of the Card Wipe transition, which is easier to use than Card Dance.
Also useful is the the Images Wall Script for After Effects by Motion Boutique, which distributes the contents of a selected folder onto a grid and provides UI parameters to control transition animation.
For some 3D, see Cinema 4D: Photo mosaic at Motionworks.