Photo duration won't change!

Hi there!

I'm trying to complete a stop-motion movie for Christmas but the duration won't change!!

I want it to be 0,5 sec, but it keeps jumping back to 0,1. No matter what I do the duration won't change!

I'm about to throw my macbook out of my window!

Apple: Could you please do something!!?

Thank you if you have any idea of what could help me!!!

PS: I'm using mavericks