First make sure image previews are active click on the desktop then CMD+J then ensure preview is enabled. Is this the case the following might help.
In case the image previews on OS X are not working anymore (only generic icons are shown) it might help to open the terminal and to execute :
rm /Users/{your user}/Library/Preferences/com.apple.finder.plist;killall Finder
This actually also helped me once Adobe Premiere Pro didn’t startup anymore (loaded like forever).