I’m not sure where the fault lies, but Adobe’s Lightroom is capable of making corrupt jpegs which can crash the Finder in Leopard. Not only this, but if one of these corrupt jpegs finds its way into your Downloads folder, it can break the Dock, Spaces, minimise (amber button) and command-tab. These are the ones I’ve discovered, there may be others.
To fix the problem, remove the corrupt jpeg from your downloads folder and trash it.
This is a nasty problem which both Apple and Adobe should be working on – Apple form the point of view that a single corrupt file can break so many essential parts of the operating system, and Adobe because their product is capable of generating corrupt jpegs.
Digging a little deeper in Adobe-related forums, they’re suggesting that it’s the Apple OS at fault, misinterpreting data embedded in jpegs. Whatever – I hope it gets fixed soon.
The problem is under discussion in the Apple Support Discussions.