To get an image to be centered horizontally and vertically, your scene would need to be structured differently. Check out this document:
image-centered.hype.zip (512.9 KB)
In this document, I have a group set to scale, with ‘shrink to fit’ and ‘zoom contents’ on the group. The scene is set to scale by 100% in both dimensions. This sets up the relationship between the size of the image and the size of the scene.
For your document, you may want to instead use something like Fancy Box or something like http://noelboss.github.io/featherlight/ to accomplish this.