I have notified the developers of this but you might like to post your suggestion in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
Thank you.
However, there should be very little visual difference between an image processed at 99% quality and the same image processed at 100% quality.
Also, I find that the law of diminishing returns usually kicks in somewhere around 90% when file size starts to rise considerably with little noticeable increase in visual quality.
JuiceboxBuilder-Pro's default value of 80% is usually a good compromise between file size and visual quality (although it will certainly depend on the images being compressed).
You could choose to resize your images in an imaging program (such as Adobe Photoshop) prior to using JuiceboxBuilder-Pro and deselect the 'Resize Images' checkbox on JuiceboxBuilder-Pro's 'Images' tab.
JuiceboxBuilder-Pro will then copy the source images across to the gallery's 'images' folder without resizing them at all.