Custom Thumbnails in Wordpress using Imagick

I want to upload an image to Wordpress and create a really custom additional thumbnail for it using imagick.

I have the code for imagick - that works. But I have no idea, how I should implement it in the wordpress upload. Can you help me with this? Is there maybe a better way to do this than to hook into the Wordpress upload event?

Thank you.

My code for imagick is:

    <?php
/* Create new object */
$im = new Imagick();

/* Create new checkerboard pattern */
$im->readImage('test.jpg'); 

/* Set the image format to png */
$im->setImageFormat('png');

/* Fill new visible areas with transparent */
$im->setImageVirtualPixelMethod(Imagick::VIRTUALPIXELMETHOD_TRANSPARENT);

/* Activate matte */
$im->setImageMatte(true);

/* Control points for the distortion */
$controlPoints = array(500, 30, 133,150,90,2800);
/*size, inner-circle, padding links, padding-top (jeweils Mittelpunkt), wie weit, */


/* Perform the distortion */                       
$im->distortImage(Imagick::DISTORTION_POLAR, $controlPoints, true);



/* Ouput the image */
header("Content-Type: image/png");
echo $im;
?>