Just a quick note: if you’ve got problems in OS X with your Finder previews not showing up, apparently fairly randomly, I may have the answer.
This was my problem: some of my JPEG (and other) previews would be fine, some would be just showing the default “it’s a JPEG” icon. Quick Look actually worked for these; it was just the preview in the Finder (including Cover Flow.)
Relaunching the Finder would seem to cure the problem, but it would come back again at some point. Existing previews seemed to work fine, but sometimes, previews of new or moved images wouldn’t work.
I tracked it down to the Gimp — but I guess any third-party application that can generate Finder previews could cause the problem. I had an old version of Gimp.app installed, version 2.6.3. Eventually I figured out that my image previews would be generated fine, but only up until the first time I pointed the Finder at a folder with a .xcf (Gimp native) graphic file in it.
I guess this is what happened: to generate their own previews, some programs register with the Finder so that when it comes across one of their files, it asks them to generate a preview. I’m fairly convinced that the version of Gimp.app I had installed had some kind of fairly horrendous bug when generating Finder previews — horrendous enough to kill the Finder preview generation for anything from the point where it saw a .xcf file.
Upgrading to Gimp.app 2.6.4 (and restarting the Finder to get the previews back working again) seems to have cured the problem. Not only that, but I now have .xcf file previews appearing in the Finder, which is the first time I’ve seen them — I guess this functionality has been added in the Mac version of the Gimp fairly recently. Which might be why it was a bit flaky to start with!
So, if you’ve got the problem, have you got any flaky/beta/suspect third-party application that might be breaking the Finder? If so, try updating them (or deleting them if you don’t use them any more), and relaunching the Finder (or just rebooting.) Let’s face it, it can’t do any harm…