I was able to unlock the last picture along with the second to last picture. It makes me think that maybe you didn't get enough points when speaking with the first attendant. Maybe try picking different answers with him until you get the full scene in the garage. I still can't figure out how to unlock that 3rd picture. My guess is you have to use the rag on the dusty clock when clicking on things in the tent, but no matter how many times I click the rag and then the clock, it just keeps telling me it hasn't been dusted in a long time. Maybe that's not even what you have to do, but I've clicked everything in that tent and based on the gallery, I think it has something to do with the tent.