Nextgen: how to disable resize

I’m using Nextgen, and I don’t want my images resized. I’ve tried leaving the width and height boxes blank, but that doesn’t seem to work (even if I delete my images and upload again).

Is there a way to disable resizing, so the full size (not thumbnail) images are the same size as the originals I uploaded?

I’ve got a bunch of images at different sizes, so if I specify pixel dimensions some images will either get reduced or enlarged.