Trick is in the z-index of the image and the black background
Check out this post I put up of how to do a “walkthrough tour” it goes over the things needed here as well, which is basically z-index
Another alternative is to just create a group with black as the background color and place the image inside of that group. Then set up conditionals to hide/show that group and have it cover the entire page.
I used the z-index in my image uploader for the cropping portion…although not a full screen black background, it still has the black background group set with a lower z-index than the croppie element.
