I am having a version of the same problem.
I initially assumed it was a problem with timthumb (hate the name BTW), as I could upload and image and it would work sometimes and thn not others.
After some trial and error I realised that it is a Chrome problem rather then anything else.
Either chrome is not sending the right URL (maybe its too long or something like) or Chrome is not understanding what is coming back.
Whats really weird is that sometimes Chrome will render an thumb it would not touch before, and some which were working suddently don't.
It will be interesting when the solution is found becuase its nuts...