Four thumbnail modes are available. None, Layout, Scaled, and Uniformity. If you're confused which mode to use, try Scaled first.
None will disable thumbnails, and uses a lot of bandwidth. Layout will keep most pictures their original dimmentions, and usually uses a vertical layout for wide pictures to keep comments readable. Scaled will use thumbnails for wide pictures, and favor horizontal layout. Uniformity makes all the pictures the same size with a small thumbnail.
If you choose “None”, thumbnail support will always be off for all members unless you enable it later in the control panel.