Technical possibilities (Score 1)

They probably use wavelets (an extended sort of Fourier Transform). Images (or sound or ..) are represented in a hierarchical wavelet-structure that allows you to query it very rapidly for certain shapes or colors.

If you crawl the web and compute the wavelet-transformations of all the pictures you encounter, you have a very fast image query at your disposal.

Wavelets are also used by the FBI for instantly veryfying the identity of a person encountered on the road by sending the wavelet-transform of his fingerprint.