Unfortunately, I'm not exactly sure what you mean.
Maybe you could explain your problem in more detail (let me know what you see and what you expect to see) and, if possible, post screenshots to demonstrate the problem.
In the meantime, maybe my notes below will help.
When using Juicebox-Pro's Background Image Options (found here), you can select the backgroundScale (STRETCH, FILL or NONE) in JuiceboxBuilder-Pro's 'Customize -> Background Image' section but the background image itself does not change (and is not adjusted) which each image within the gallery is viewed.
You can set your gallery's backgroundColor to be transparent (set the Background Color Opacity to '0' in JuiceboxBuilder-Pro's 'Customize -> Lite' section) and whatever CSS background you set in your gallery's embedding web page (whether for the gallery container or the entire web page) will shine through the gallery.
If you wanted to, you could then adjust (or change) the background depending on the image currently being displayed in the gallery (using JavaScript and CSS).
Here is an example which changes the gallery's background color to red for a portrait style image, green for a landscape style image and blue for a perfectly square image. You could modify the JavaScript to alter the background and way you like, for example to completely change or adjust the size of a CSS background image.
To see this example in action, create a sample gallery with JuiceboxBuilder-Pro and use the following as the gallery's 'index.html' file.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta name="viewport" id="jb-viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=0" />
<style type="text/css">
body {
margin: 0px;
}
</style>
<script type="text/javascript" src="jbcore/juicebox.js"></script>
<script type="text/javascript">
var jb = new juicebox({
backgroundColor: "rgba(0, 0, 0, 0)",
containerId: "juicebox-container"
});
jb.onImageChange = function(e) {
var index = e.id;
var info = jb.getImageInfo(index);
var url = info.imageURL;
var img = new Image();
img.src = url;
var imageHeight = img.height;
var imageWidth = img.width;
var imageRatio = imageHeight/imageWidth;
if (imageRatio > 1) {
// Image is portrait style
$('#juicebox-container').css('background-color', '#ff0000');
}
if (imageRatio < 1) {
// Image is landscape style
$('#juicebox-container').css('background-color', '#00ff00');
}
if (imageRatio === 1) {
// Image is perfectly square
$('#juicebox-container').css('background-color', '#0000ff');
}
};
</script>
<title>Test</title>
</head>
<body>
<div id="juicebox-container"></div>
</body>
</html>
I hope this help to clarify things and point you in the right direction.
If not, then please explain your problem in greater detail and I'll try to help further.
Thank you.