I agree with the other posters though that it needs to be a bit more forgiving with spelling and adjectives. "Yellow Car" and "Red Apple" should both be accepted. I never saw Big Bird but if you decided to keep it, "bird" and "yellow bird" should be correct unfamiliar users.

I tried to use tineye.com to see if a spammer could automate searches to determine what the picture is of (obviously wouldn't work on all pictures though) and it failed for 'car' and 'pencil'. Google Googles on my phone identified the bird as a Lark: http://www.tropicalbirding.com/tripReports/TR_SouthIndia_Nov... which was interesting but not a correct answer.