The buttonBarPosition configuration option allows you to position the Button Bar in the TOP area but this applies only to Large Screen Mode (there is no TOP area in Small Screen Mode where the Button Bar is always positioned on the overlay).
However, if you force Large Screen Mode to always be used on all devices and in all browsers (by setting screenMode="LARGE"), then the TOP area will still exist when the gallery is displayed on mobile devices and setting buttonBarPosition="TOP" should solve your problem.
No matter whether your gallery is being displayed in Small Screen Mode or Large Screen Mode, if you tap the main image (on a touch-enabled mobile device), the overlay (including the Button Bar and the caption area) will disappear (to leave just the image). Tapping the main image again will make the overlay reappear.
You can check it out in this demo gallery (when viewed on a mobile device): https://juicebox.net/demos/pro/default/
Please note that tapping the main image will not make the overlay disappear (in Small Screen Mode on a mobile device) if either:
(1) You set showInfoButton="TRUE" where tapping the Info Button on the Button Bar will make all of the overlay disappear (except for the Button Bar, which needs to be kept visible at all times so that the user can tap the Info Button again to make the rest of the overlay reappear).
... or:
(2) You set imageClickMode="OPEN_URL" where tapping the main image will open the image (or associated linkURL) in a new page of its own.
Essentially, if you are already setting screenMode="LARGE", then just set buttonBarPosition="TOP" to position the Button Bar in the reserved TOP area (where it will not overlap the main image).
Otherwise, in Touch Input Mode (on mobile devices), tapping the main image will toggle the overlay on and off (with the exceptions I mentioned above).
I hope this helps.