there is a template functin printCustomSizedImage that you would use in place of $image->getThumb(). You would have to have:
`global $_zp_current_image;
$_zp_current_image = $image;
printCustomSizedImage($alt, $size, $width, $height, $cropw, $croph, $cropx, $cropy, $class, $id);`
to print the image.