Thumbnails for Image Files on Desktop won't load

Based on your log, I suggest trashing the Finder property list, then rebooting.

• Open Terminal
• Enter the following command rm ~/Library/Preferences/com.apple.finder.plist
• Restart
• If the issue persists, I suggest following these instructions. to completely reset the service.

Further information: The X Lab's Maverick's Post Installation Issues

OMG that linked gist did solve it! (from Apple forums).

Here is the bit that worked and fixed it:

• mkdir ~/tmp-quicklook
• mv /System/Library/LaunchAgents/com.apple.quicklook.* ~/tmp-quicklook/
• Log out/Log in (or reboot)
• sudo mv ~/tmp-quicklook/com.apple.quicklook.* /System/Library/LaunchAgents
• Log out/log in (or reboot)

(the two "resets" are really necessary)

Still the folder preview doesn't work but looks like it's much harder to fix: