Reset QuickLook Thumbnail Cache: Open Terminal and run the following commands

gistfile1.sh

sudo find /var/folders/ -name com.apple.QuickLook.thumbnailcache -type d
# Enter your password and hit enter (it will not display your password).
cd COPY-AND-PAST-THE-ABOVE-OUTPUT-HERE
cd ../
sudo mv com.apple.QuickLook.thumbnailcache com.apple.QuickLook.thumbnailcache-BACK
# Restart your computer.


### If anything goes wrong run the following:
sudo find /var/folders/ -name com.apple.QuickLook.thumbnailcache-BACK -type d
cd COPY-AND-PAST-THE-ABOVE-OUTPUT-HERE
cd ../
sudo mv com.apple.QuickLook.thumbnailcache-BACK com.apple.QuickLook.thumbnailcache

There's no need for that, QuickLook has a command for it:

qlmanage -r cache