Knowledge Checks when using 360 degree images

I'm creating my first 360 degree image course and I would like to have one of the triggers pull up a question on top of the 360 degree image.

I have seen an example of this but when I try it myself using a light box a majority of the slide is just blackened out rather than see through/transparent.

I guess what I needs is for the question slide to be fully transparent but I've tried adjusting the background and a number of other things but to no avail.

If only you could add a question to a layer then I would get the functionality I need. 

Any thoughts or ideas would be appreciated.