You could perhaps increase the stagePadding (in JuiceboxBuilder-Pro's 'Customize -> General' section) to introduce a certain amount of padding all around the gallery) and reduce the height of the caption area, using maxCaptionHeight ('Customize -> Caption'). (As your gallery uses captionPosition="BOTTOM", the maxCaptionHeight will be the actual height of the caption area.)
Otherwise, you could reserve enough space at the top of your web page for your logo (in a header area) and display the gallery in the remainder of the browser window. Please see the View Resizable Gallery with Top Menu Example in the Resizable Gallery support section for an example of how this can be done. You can view the source of the web page in a browser and copy /modify the code to suit your own needs (removing the footer, replacing the content of the header with your own logo and swapping the sample gallery for your own).
Alternatively, there are other suggestions regarding how and where to place a logo in a Juicebox gallery in the Adding a Logo support section. One possible solution would be to use your logo in the Gallery Title and then set galleryTitlePosition="TOP" and galleryTitleHAlign="CENTER".