Joomla 2.5: Where Does JToolBarHelper::custom Load Icon?

It's loaded from: administrator/templates/<selected template>/images/toolbar

For example: create a NewButton on the toolbar of your component
1. Copy ButtonIcon into administrator/templates/<selected template>/images/toolbar floder
2. Add new line to administrator/templates/<selected template>/css/template.css
.icon-32-ButtonIcon { background-image: url(../images/toolbar/icon-32-ButtonIcon.png); }
3. Declare it in your code:
JToolBarHelper::custom( 'yourtask', 'ButtonIcon.png', ' ButtonIcon .png', 'Button Name', false,  false );