OPTION A (single slide):
Become best friends with the Selection Pane and name every image. Only view the ones you wish to work with at any given moment.

OPTION 2 (multiple slides--better option I think and more likely to render animation smoothly):
This option gives the illusion that you are retaining all the pictures, but you are only working with 4 at a time across multiple slides.
1) Create the first slide with 4 pictures. We'll call this slide A.
2) Save the completed slide as an image as follows:
Click File<Save As and change file type to .png
Save the file (e.g. "photo collage 1.png"). When prompted, select "This slide only"
3)Create a new blank slide--we'll call this slide B. Format the background to be "Picture or Texture fill" and browse to select the image you just saved. So now slide B looks like the completed version of slide A.
4) Add four new photos to slide B.
5) Repeat steps 2-5 until complete. With 100 photos, this will give you a total of 25 image files/slides to work with, but only 4 animated photos per slide.