Preloader Position

I am using the Default preloader for a HTML5 animation to have something on the screen during the transition between two Animate files.

The first published Animate's .html page links to another and there is a lag while the second one loads. This jump from one HTML file to another happens right after "The following six patients arrive at your Emergency Department(ED)."

Depending the screen size, the default throbber GIF shows in the corner or not at all. I would love for it to be centered horizontally and vertically.

My Publish Settings are set to "Make Responsive" with Center Stage unchecked. With Center Stage Checked it does center horizonatally, but not vertically.

Is there a setting I am missing?

Also, I would like to create my own preloader, is there a certain size I should make it?

My animate piece is sized 1920x1080.