Unfortunately, Google bought out the ESP game, and then after shut them down.
Btw, loading a single gif url, like http://gifgif.media.mit.edu/gif/13AbwdfEHTvih2 is too slow. dev console shows "Cannot read property 'num_votes' from undefined". You need to prepare the template code for receiving an empty collection while the subs fill the data, i normally shortcut it as Collection.findOne({id: foo}) && Collection.findOne({id:foo}).num_votes. to prevent your error.
Also, some waitOn and loading templates on iron-router will make the overall UX much better with few changes. specially on the results page that is empty for a long time until content kicks in.
Regarding the site concept, its fun. I spent too much time there already. I will show it to my eve game mates. At eve chats the text line to "emotional" gif ratio is about 1:1. Good job.
Sure you can add more, but how many can you add before users get upset about doing the same thing over and over again? Three? Four? At four times I'm still looking at a 6% chance of getting an account created if I just select randomly. That's plenty.
Suggestions to use things that humans can easily distinguish but computers can't as captchas come up often. However, being only human readable isn't the only requirement for a captcha - another requirement is that it can't be computer guessable. If a computer can guess correctly with a high (>1%) chance of success, then the captcha is pointless.
I had a teacher who employed "multiple multiple choice" questions on his exams. The question would have some number of choices (e.g. a,b,c,d,e) and you had to choose the correct answers. These questions were tricky because the answer choices could all be wrong (meaning you select none of the answer choices), could all be correct, and any combination of the answers could be correct. It made guessing go from a 1 in 5 situation to 1 in 32. I was not a fan.