But like the title says, this is an illusion. In fact, both halves of the image are identical. In this Instructable we'll quickly go over a way to make one of these images on your own.
Step 1: Get a Picture
Look for pictures where the perspective is from below so that there is a distinct difference in the angles of the sides of the building. This gives a strong feeling of where the lines would converge. The second image shows where the vanishing point is.
As such, you want a building with straight sides. Good keywords to search for are "building," "up," and "perspective."
The source image can be found here.
Step 2: Crop It
Step 3: Double Up
Inspiration for this illusion from The Leaning tower Illusion by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu.