Mavericks Dock icons have become corrupted. Is there a cache to flush or a database to rebuild?
In case you don't want to use some "cache cleaner app" then just try doing it manually from the terminal with these three commands followed by a reboot:

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

or

Alternatively a single boot into Safe Mode once could/should clear out some caches.

Worked For Me™ in Sierra (10.12)