Preloading can help, but note that it is more pre-[down]loading, and the browser typically still needs to decode and render the image. Thus if is large in pixel dimensions can lead to the page being held up.
A simple test would be to try unchecking “Use WebKit graphics acceleration” and/or checking “Position with CSS left/top” in the Document Inspector. This changes the rendering characteristics and might help.
Otherwise without seeing the .hype document it’d be hard to provide any other advice!