Babe photos that need unlocking

Just wanted to ask what aspects of a photo decide whether non-paying members have to use credits to unlock them?

I thought it was just the hardcore photos but then I noticed one of my uploads - this one - has to be unlocked and it's not a hardcore shot at all.

The "close up" category also needs to be unlocked, and this picture seems to have made it into 'that category. I'm going to go ahead and change it's category as I don't see a need for it to be there.