How to make a LEGO mosaic
LEGO mosaics can be realized in various ways. You can start with processing a color image, scale it down in size and reduce the number of colors to match the ones available in the LEGO palette. The simplest way to build a LEGO mosaic is to attach many colored 1×1 tiles or 1×2 LEGO plates on a large LEGO baseplate, getting a square pixel aspect ratio. An alternative is to use plates with studs up, getting a different pixel aspect ratio, because LEGO plates are shorter than wider.
A double LEGO mosaic
There is a way to make a LEGO mosaic that shows two pictures in one: it’s called a lenticular mosaic, and involves using LEGO 1×1 slope bricks instead of plates or tiles. The design process is slightly more complex, because you have to start with two images, split them in columns (or rows) and then assemble each row interleaving one image with the other. See the video to understand better how this process works. The result is a picture that is confusing, if seen from front, but that shows the two original images when seen from sideways.