I expect you might be referring to the image used for the Splash Page on mobile devices.

The Splash Page is a placeholder for the gallery which is displayed by default on small screen devices when the gallery is embedded in a page alongside other content (rather than displayed on a page of its own with dimensions of 100% x 100%, filling the browser window) and may may too small to be usable.
The Splash Page is essentially an image link for the gallery which displays an image (by default, the first image in the gallery) and some text.

When the user clicks or taps the Splash Page, the gallery is expanded to fill the user's browser window (giving the images more space to be displayed). If you set showSmallThumbsOnLoad="FALSE" (in the 'Thumbnail' section), then, when the Splash Page is clicked or tapped in Small Screen Mode (on mobile devices), the firstImageIndex image (rather than the thumbnail page) will initially be displayed when the gallery is expanded.

By default, the Splash Page uses the first image in the gallery and the image is resized to fill the Splash Page (cropping may occur).
You can choose a different image to represent your gallery on the Splash Page using the splashImageUrl configuration option.
You can further customize the Splash Page using the Splash Page configuration options.

For more information about Screen Modes, the Splash Page and how Juicebox adapts to different devices and screen sizes, please see here: http://www.juicebox.net/tour/gallery/

You can choose to not use the Splash Page by setting showSplashPage="NEVER" (in the 'Splash Page' section).
Alternatively, you can force the gallery to be displayed in Large Screen Mode (which, by default, does not use the Splash Page) on all devices and in all browsers by setting screenMode="LARGE" (in the 'General' section).

I hope this helps.