Finder folder icons look like document and image files
Try resetting the Launch Services database. Dig into your Utilities folder and paste this into Terminal:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user


You may need to restart the Dock and Finder to see if it's worked:

killall Dock; killall Finder


Alternatively, just reboot after you've reset the Launch Services database.