Creating a mosaic of images

Hello there! I'm trying to do something that is at a coding level beyond my skill and I'm hoping for a few pointers.

I want to make something that looks for when new images enter a folder, then when one does, displays that image really big, then fades it back into a mosaic of all the rest of the images in the folder. They don't need to make a pattern or anything. It didn't seem like it would be that hard at first, but I'm really stuck. Any thoughts? Thanks!