Actually, if you do a google image search and actually look at SnapChat's "CAPTCHA", it's unbelievable, like a piece of work from the nineties.
It shows you nine images and asks you to select the ones where the ghost appears. (Random selections net 1 success in 512 right there, and they probably won't show you zero, one, eight, or nine ghosts, increasing success rates to 492 to 1.)
Notice that a ghost or its impostor is always the only white shape in the image. (Sometimes there are also a few white stars, moons, etc.) To improve from random guessing, isolate the white blob, select its center of mass, transform the outline into polar coordinates, perform a Fourier transform, prepare a vector from the Fourier coefficients, and all the ghosts will cluster together in that vector space. (There will also be a star cluster, an apple cluster, a tree trunk cluster, a top hat cluster, a full moon cluster, etc.)