The screenshot window displays thumbnails of the nine screenshots taken over the two second span around the time that you clicked the "Take Screenshot" button. You can select a thumbnail to inspect it in the larger view to the left.
Allows you to save the currently highlighted screenshot. You can choose the destination file format from the pop-up in the file save dialogue. .tiff, .jpeg and .png formats are currently supported. iTunes Connect currently allows only .tiff and .jpeg images to be used as screenshots in the App Store.