The module is style agnostic in its implementation; it only creates HTML markup. You could use either images or icon fonts in your theme's CSS, or install a module such as Font Awesome Icons to provide the styles to actually show the icons.
The module has configurable settings that allow you to customize it depending on the naming convention your icon classes expect:
- Class prefix: the prefix for the icon itself. i.e 'icon-', 'fa fa-', 'glyphicon glyphicon-'
- Icon size prefix: the prefix for the icon sizing classes: i.e 'icon-', 'fa-'
- Icon size: the size that you want the icons to be displayed at. Sizes provided: 1x (default, use this to omit the size class), lg, 2x, 3x, 4x, 5x
Sandbox Project (drupal.org)