Thumbnail shows different image from the slide

The thumbnail of a slide comes from its Thumbnail option. This is the thumbnail you are seeing on the slide list or at controls, like the Thumbnail control.

The way this thumbnail is generated, that when you create a slide, by default its Background image's url will be inserted into the Thumbnail field. 

When the background image is changed, and its url matches the thumbnail image url (which matches by default), then we are changing the thumbnail image automatically. If this doesn't happens at you, you have a different background image url from the thumbnail image's url!