Image style generating hook
The image style hooks are documented here:

http://api.drupal.org/api/drupal/modules%21image%21image.api.php/7


hook_image_default_styles:     Provide module-based image styles for reuse throughout Drupal.
hook_image_effect_info:    Define information about image effects provided by a module.
hook_image_effect_info_alter:  Alter the information provided in 
hook_image_effect_info.
hook_image_styles_alter:   Modify any image styles provided by other modules or the user.
hook_image_style_delete:   Respond to image style deletion.
hook_image_style_flush:    Respond to image style flushing.
hook_image_style_save:     Respond to image style updating.


Nothing for the actual image generation process, if that's what you're looking for.

See image_style_deliver for the details on the image generation