WordPress 3.8 introduced a new Admin design which now uses Font Icons instead of images. This method has many benefits such as great support for Retina devices and quicker load times.

Before WordPress 3.8 you had to use images for your icons, now you can use Font Icons, specifically the icons designed for WordPress called " Dashicons".

A full list of Dashicons can be found here: http://melchoyce.github.io/dashicons/

To get Font Icons working for your Custom Post Type, simple set the "menu_icon" argument along with the icon class name when you are registering your post type. An example of this code is below:

If you cannot view this code snippet properly, please click here.