Snapchat has a little spam problem.
For months now, spam profiles have been sending out nudie pictures of women with requests to add their "Kik" account, which, obviously, isn't a real account. So, in order to combat this problem, Snapchat has introduced a feature for new users that makes them play a "game" before signing up. When a user attempts to create an account, they'll be asked to find the ghost (the Snapchat logo) in a series of pictures. As long as they find the right ones, they'll be able to continue. If they can't find the ghosts, Snapchat will consider them a spambot and they'll be out of luck. It's pretty similar to the captcha feature you get on most websites while logging in.
Now that Snapchat is attempting to fix their spam problem, hopefully that means they have their security under control.