Store the values passed to a toolkit call.
$op: One of the image toolkit operations: 'get_info', 'load', 'save', 'settings', 'resize', 'rotate', 'crop', 'desaturate'.
$args: Values passed to hook.
function _image_test_log_call($op, $args) {
$results = variable_get('image_test_results', array());
$results[$op][] = $args;
variable_set('image_test_results', $results);
}