Continue to lay out your boxes, adding in some smaller ones for vertical images. I also like to use guides to make sure everything lines up.
This is just good housekeeping, to name the layers. And it makes it easier to remember which picture goes where.
Now it is time to start placing your images! I just dragged and dropped this picture of Loki from Windows Explorer, but you can also go File > Place.
Right Click on the image's layer and click "Create clipping mask"
And it clips the image to the shape of the shape below it! (But we aren't done yet)
Add in some more pictures... (I just added Loki being mischievous.. as usual.)