Juicebox-Lite uses the default value for imageScaleMode (SCALE_DOWN) whereby large images are scaled down to fit within the gallery's image area and be displayed as large as possible whilst respecting their aspect ratios and without cropping (but small images are not scaled up).
Make sure that your gallery images are large enough in terms of resolution (larger than the gallery) and your images will fill the gallery's image area top to bottom or left to right, depending on whether the images are landscape or portrait orientation.

With Juicebox-Pro, you can set imageScaleMode to other values such as SCALE, which scales up small images (as well as scaling down large images) but this is likely to reduce the visual quality of the images so using larger gallery images is usually a better solution.

For reference, short descriptions for all possible values of imageScaleMode can be found in the Main Image section of the Config Options page.