I have positioned an image at the center of a page.

It's absolute positioning.

Above it is an absolutely positioned header.

When I remove the absolute positioning of the header, the image moves down.

It shouldn't do that. It would mean it's not positioned absolute.

But I put position: absolute; into the Css of that image's TIFF.

Why does it behave so weird?

Absolute positioning will take it's position from the first positioned ancestor OR if there is no positioned ancestor, it will be positioned relative to the body element.