Add External Image option for Backgrounds

gistfile1.txt

extend the Group Control Base in my plugin to create an external background image option field to go with the standard classic background options.

class Group_Control_Background_trchristensen extends Group_Control_Background {
    public function init_fields() {
        $fields = parent::init_fields();
        //here you add your field
        $fields['ext_image'] = [
                'label' => _x( 'External Image', 'Background Control', 'elementor' ),
                'type' => Controls_Manager::TEXT,
                'title' => _x( 'Background Image', 'Background Control', 'elementor' ),
                'selectors' => [
                      '{{SELECTOR}}' => 'background-image: url("{{URL}}");',
                ],
                'condition' => [
                        'background' => [ 'classic' ],
                ],
         ];
         return $fields;
    }
}