image derivative

Hi this seems to be fine on the first look.

BUt you should wrap the whole derivate options form block inside an
to prevent errors on installations where the image.module isnt installed

k inline < ?php tags wont work... good to know:
----
[...]înside an

if (module_exists('image')) { 
  $options = array();
  $sizes = _image_get_sizes
  ...
}

to prevent errors on installations where the image.module isnt installed

Committed to HEAD, please try it out and report back here.