If you mean a photo mosiac where you have a bunch of images that are laid out to make an image that looks like another image, there's no one click solution for that in After Effects. There are a bunch of apps out there that will take a folder full of images and generate a photo mosiac from them based on a master photo. There's nothing that I know of that will write that image as separate layers in a PSD file so you could animate them. You can, however, use card dance to break up the image in 3D and fly the pieces out or in to make up the image.
I'd try a google search for PhotoMosiac. There's a free app for a Mac, which I've never used, and a few sites where they will make you one online. You can also fake it by creating a pattern in Photoshop with some smaller images, desaturating the pattern and then filling a layer in overlay or soft light to get the effect.
Hope this points you in the right direction. You might want to take a look at this old tutorial for AE6 from Ayato@web