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