It's set at install time by the installer, either from its own predefined list or from the data returned from the Creative Commons license chooser.
Generally it will be of the form:
$wgRightsIcon = "$wgScriptPath/resources/assets/licenses/cc-by-sa.png";
$wgRightsIcon = "$wgStylePath/common/images/cc-by-sa.png";
The icons available by default are listed in the File column of this table: