If you use Juicebox-Pro's own Background Image Options (and your background image fills the gallery area without any transparency), then you may see a brief flash of color (grey by default) before the background image is displayed.
This is because the backgroundColor (set in the gallery's embedding code) is read and processed before any configuration options in the gallery's 'config.xml' file (such as backgroundUrl).
Set the gallery's backgroundColor to be completely transparent (any color with an opacity of 0) and this should resolve this issue.

backgroundColor: "rgba(0,0,0,0)"

(You can change the backgroundColor in JuiceboxBuilder-Pro's 'Customize -> Lite' section or by manually editing the gallery's embedding code.)

Incidentally, if the background image does not fill the gallery area (if backgroundScale="NONE") or if the background image is a PNG file with transparency, then the backgroundColor still needs to be active.