Thumbnail size changes when set to [none]

This is a CSS issue – it looks like you have “.gallery-icon a img {width:100%;}” in your CSS, and when I disable the width:100%;, it scales down to the same size as the image with no link. You’ll need to change your css to be something like “.gallery-icon a img, .gallery-icon img {width:100%;}”. There may be other instances in your css where the gallery is expecting links, in which case you’d need to modify it to also support images with no links.