Today, I was working with someone with OS X 10.6. When she wanted to pick up an imported image, it was not possible. I transferred the project and the image to my computer with OS X 10.9; there it worked.
Is this a known bug? Or a feature that is not supported on older versions of OS X?
What version of Glyphs was running on 10.6?
Ah, sorry to forgot that. The latest stable release was running, 1.4.3.
There are quite a few problems with 10.6. I strongly suggest upgrading to 10.7+, the newer the better. Are you stuck with 10.6 on that machine?
Luckily no 
There are some troubles with a blocked Apple-ID, but I have also suggested to update when that is solved.
Will do so when Apple helps us, and see what the upgrade gives!