Just increase the inactivityTimeout value from its default value of '4' (seconds) in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.
Alternatively, you can have the overlay never auto-hide by either setting inactivityTimeout to '0' or showImageOverlay (also in the 'Main Image' section) to 'ALWAYS'.
You could also display the Info Button on the gallery's Button Bar by setting showInfoButton to 'TRUE' (in the 'Button Bar' section) to allow visitors to your website to toggle the overlay on and off (by clicking the Info Button).
For reference, short descriptions of all configuration options can be found on the Config Options page.