Try `killall -KILL Finder` in Terminal. Usually clears up graphical glitches.

Try resetting Finder preferences. Run these commands in Terminal:

    rm ~/Library/Preferences/com.apple.finder.plist
    killall -KILL Finder

Note that you will have to redo any Finder preferences you had set.

If that doesn't work, follow [these steps](http://furbo.org/2015/01/19/clearing-the-icon-services-cache-in-yosemite/) to reset the Icon Services cache:

    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

    killall -KILL Dock
    killall -KILL Finder

After the first command, it will prompt you for a password. Enter your login password.