When expanding a Juicebox-Lite gallery, the main image used in the expanded gallery is exactly the same one that is used in the normal (unexpanded) gallery - the image defined by the imageURL in the gallery's XML file.
Also, in Juicebox-Lite, the imageScaleMode configuration option is set its default value of SCALE_DOWN. This will dynamically scale down large images to fit within the gallery's image area but will not scale up smaller images.
Therefore, the images in your gallery should be large enough to fill the image area in the expanded gallery. In the normal (unexpanded) gallery, Juicebox will just shrink them down a bit to fit (your images will not be cropped).
If you were to purchase Juicebox-Pro, you would be able to change the value of imageScaleMode to SCALE_DOWN, SCALE, FILL, STRETCH or NONE. Descriptions of these values can be found in the Main Image section of the Config Options page.
Also, with Juicebox-Pro, you would be able to have a Multi-Size Image gallery whereby you would be able to have three different image sizes in your gallery and Juicebox would determine the best image size to be displayed depending on the device, browser window size and whether or not the gallery is expanded. For example, in Large Screen Mode, 'medium' images would be used in the normal (unexpanded) gallery and 'large' images would be used in the expanded gallery. (You can choose the dimensions for all three image sizes in JuiceboxBuilder-Pro.) For more information about Multi-Size Image Support, please see this blog entry.
Can you share its config.xml?
The Lite Embedded demo gallery's 'config.xml' file can be found here (although the information above may be more useful):
http://www.juicebox.net/demos/lite/full/config.xml