I would take the final graphic and work backwards in Photoshop. Erasing a portion that represents one pic at a time so I end up with 80 bew pics. Then lay all 81 new background pics on the Vegas timeline on Track 2. When palyed, it would appear as the final mosiac pic grows with each new portion popping on. Now lay all of the full size images on Track 1 and keyframe them as needed. Adjust durations as needed.
In Photoshop, take the final image, make it a layer, create a new layer below it the color of the background (whatever it might be). Erase one small portion that represents one of the 80 pics, save it as number 80, erase another, save it as number 79 and so on.
Gary