Open the General Preferences and check the Double-click opens snapshot image clips in: checkbox. Then click the Select Application... button and select the application you want the snapshot images to open in when you double-click on a thumbnail in the Snapshot Clips Control Panel.
If no application is selected in the General Preferences, then double clicking on the thumbnail will cause the image to open in the system's default image viewing application, usually Preview.
To open multiple snapshot images from the Snapshot Clips Control Panel, select the snapshot image thumbnails you want to open using the Shift or Command key. Then hold the Shift or Command key down while double-clicking in one of the already selected thumbnails.
Note: Each time a snapshot is taken, a thumbnail of the snapshot image will be created and added to the Snapshot Clips Control Panel.