thanks for your reaction Steven although I am disappointed there is no solution for the problem. As a longtime user of  SV-Builder pro I am used to this way of making a slider. I bought juicebox because it was not easy (I could not manage) to implement SV-Builder into WordPress. Hope this will be better with the Juicebox WP-Plugin.
For now I will solve the problem by chancing the background of my images to the color I want and make the background in JuiceBox the same color. And try to implement with the WP-Juicebox plugin.
Remko