The image you are describing is the Splash Page image.
More information about the Splash Page can be found in the Screen Modes section of the Gallery Tour.
By default, the Slash Page uses the first image in the gallery and scales the image to fill (rather than fit within) the Splash Page (which uses the gallery dimensions). If the image does not have the same aspect ratio as the gallery/Splash Page, then cropping will occur.
There is no way to change the scaling behavior for the Splash Page image but you can choose an image to be used for the Splash Page via the splashImageUrl configuration option.
The splashImageUrl can be an absolute path or a relative path (relative to the web page containing the gallery's embedding code).
The splashImageUrl does not need to be a gallery image so you could perhaps prepare an image that represents your gallery and looks better at the gallery/Splash Page dimensions.
For reference, all the Splash Page configuration options can be found here.