Yosemite: Corrupt quick look images
This may help : http://encase-forensic-blog.guidancesoftware.com/2014/05/examination-of-mac-os-x-quick-look.html or https://discussions.apple.com/thread/1289781?tstart=0

You should be able to rebuild the so called ThumbnailCache on your mac.
This should work. hope this helps :)