In the Client Panel, click on the "Images Settings" tab: 

For information on how to access your client panel, see this article.

"Visible Always" - The image will be visible in both the tour and available for download in your Download Center

"Visible in Gallery Only" - The image won't be visible on your tour but will be visible in the Download Center

"Never Visible" - The image won't be visible on your tour but will be visible in the Download Center 

Your edits will auto save as soon you as you click outside of the text box!