There is no way of making an image fill the page when you compile to PDF.
A 500x740 image will fill the screen on a kindle device very nicely when you output to .mobi.
But the same image only fills the left hand corner when you compile to PDF.
I have experimented with resizing the image, but at about 540 width it goes onto the next page (despite not nearly covering the width of the page). And similartly with the length.
When you compile the image to PDF the cover image looks odd. Like you haven’t bothered to centre it. It looks sloppy.
How about another option in the
Edit > Insert Image from File
Edit > Insert Image from File and Fill Screen
So that if you want to fill that particular screen (in whatever way you wanted to compile it) on that page, then you can let the program know as you insert it.
That would work great for cover images, and would also allow more flexibility for internal images, because you could forget about having to change it based on the format you are compiling to.