Another perhaps easier way to do this is to start with one image that has your background & all the small images on it.
Move just your first image off screen.
Copy & Paste this Slide for however many small images you have, for example 20.
Make sure that you have a quick transition 0 seconds between each Slide.
Then just move 1 small image off on each Slide.
You may find this easier to control as you won't have to worry about having lots of KeyFrames on 1 Slide & worrying about the timing of each KeyFrame.
Just make sure that you select the correct small image on each.
You can delete the small images from the Slide once you have moved them off.
So Slide 1 would have 20 small images & you move number 1 off
Quick Transition to Slide 2 which has 19 small images & you move number 2 off
Quick Transition to Slide 3 which has 18 small images & you move number 3 off
Continue until you have moved all the small images revealing your background
Hope this helps
Jill