Hi Seb. That's a strange one. I don't know much about OSX. Can you tell us what version you are running?
What are your GPU settings in Preferences > Performance? You could try turning off 'Use Graphics Processor' to rule it out.
I hesitate to suggest restting Preferences at this stage, as it is a PITA putting things back to how you want them, and I am not so sure it would help anyway. GPU is still my best guess, but I am not a Mac user.
I think that this is the exact same problem that I'm having. It just started yesterday. Everything was working fine, and then suddenly every photo that I open just shows a small sliver of the image at the top of the screen. The only way that I can see it is to magnify a bunch of times and even then, it only shows about half of the image.