Alter the information provided in hook_image_effect_info().
$effects: The array of image effects, keyed on the machine-readable effect name.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_image_effect_info_alter(&$effects) {
// Override the Image module's crop effect with more options.
$effects['image_crop']['effect callback'] = 'mymodule_crop_effect';
$effects['image_crop']['dimensions callback'] = 'mymodule_crop_dimensions';
$effects['image_crop']['form callback'] = 'mymodule_crop_form';
}