The image used for the Splash Page is not stretched. Rather it is scaled-to-fill the area required. The image's aspect ratio is respected (the image is not distorted) but if the aspect ratio of the image does not match that of the Splash Page area itself, then cropping will occur.
Other possible options (which are currently not available) might be to stretch the image to fill the area (which may distort the image) or scale-to-fit (which may result in blank space at the top and bottom or to the left and right of the image, depending on the aspect ratios in question).
If you would like to make any suggestions for future versions of Juicebox, then please post them in the Feature Requests forum thread. This keeps them all together and ensures that they are not overlooked. Thank you.