function reverse_srcset_order($atts) {
    if ($atts['srcset']) {
        $srcset = explode(', ', $atts['srcset']);
        $srcset_reversed = array_reverse($srcset);
        $atts['srcset'] = join(', ', $srcset_reversed);
    }
	return $atts;
}
add_filter('wp_get_attachment_image_attributes','reverse_srcset_order',1,1);