I'm an AE beginner and I'm trying to achieve a very easy effect but don't know how: an image is revealed by having squares in a grid (or mosaics, or whatever) appear randomly. The squares don't move around, they simply appear randomly until the entire image is revealed. Any ideas?
I would recommend creating a solid layer in a new composition and adding the Noise & Grain > Fractal Noise effect to it. Change the settings so that you have a series of blocks:
Fractal Type - Basic (some others may work)
Noise Type - Block
Complexity - 1.0
Then place this composition inside the composition with your image (where the transition will occur). Add a Transition > Gradient Wipe to the image. For the Gradient Layer choose the blocky pre-composition you just placed here, for Gradient Placement you'll probably find that 'Stretch Gradient to Fit' or 'Center Gradient' will work.
Now you can animate the Transition Completion from 100% (the image is invisible) down to 0% (the image is fully visible). You can set the Transition Softness to some percentage if you want each square to fade in, a higher percentage means a longer fade.
Gradient Wipe is an awesome transition you can use to control the reveal of an image using a grayscale map. I'd recommend playing with the fractal noise composition to see what else you can do!