I think you mean you don't want to have additional images resized, right? You can't disable image handler partly. The resizing happens on page request so if you want to prevent images from being resized, you need to do it on page request. Actually no resizing at all is happening when you upload images. Never. Why do you want to disable it for additional images? I can't think of any reason but that's just me and I'd really like to know.

If you really want to do this, have a look at /includes/extra_configures/bmz_image_handler_conf.php and you'll find two settings:

Code:
$ihConf['noresize_key']         = 'noresize';         //files which contain this string will not be resized
$ihConf['noresize_dirs']        = array('noresize', 'banners'); //images in directories with these names within the images directory will not be resized.
The first one means, you can add the noresize keyword to files you don't want to have resized. All you do is name your file e.g. flower_additional02_noresize.jpg (for a default image called flower.jpg) and your good. The second setting means you can upload the images to a noresize folder and the images won't be resized. You can only use the first way to prevent resizing in your case, because default and additional images need to be in the same folder.

Cheers,
Tim