A localhost WordPress could not display images from the themes folder in Xampp on a Linux Mint system.
Solution: all folders must be owned by the Apache user and group (daemon:daemon), or:
# chown -R daemon:daemon [theWordPressThemenameFolder/]
# chmod -R 777 [theWordPressThemenameFolder/]
However this is only for development purposes. On a server work by these guidelines.