Need help for converting a field formatter into a gallery preset

Hi, I am trying to convert a field formatter I've written to a gallery preset and I simple don't get it^^

right now, it 'should' output a multiple filefield with image in a kind of collage. all images will be arranged in a rectangle. you can see an example here: http://loomarea.com/drupal/Lite/node/1

unfortunately it needs exactly the same input as a normal filefield.

Please help me. I would contribute it to this module, if you like it.