QuickLook blanks when displaying some images
This "blank Quicklook image" Bug is caused by the activation of the undocumented TextSelection-Feature. 
As soon as I deleted that feature with this terminal command the blank images were never seen again ;-) 

defaults delete com.apple.finder QLEnableTextSelection; killall Finder;


If this delete-command doesn't work for you, you could try to disable the feature with 

defaults write com.apple.finder QLEnableTextSelection -boolean NO; killall Finder;