Apple - How do I restore icons in Finder?

You can reset OS X's icon cache with the following commands. Just copy and paste each into a Terminal window and then reboot your Mac.

 sudo find /private/var/folders/ -name com.apple.dock.iconcache -exec rm {} \;
 sudo find /private/var/folders/ -name com.apple.iconservices -exec rm -rf {} \;
 sudo rm -rf /Library/Caches/com.apple.iconservices.store

Source: Clearing the Icon Services cache in Yosemite