Make the image the size of the Screen.
e.g. iBook for iPad the images size need to be 600x860pixels. This will fill up the screen.
I guess you could have something like making the image sit at a drop of 50%.
p.chapterhead {
margin-top: 50%;
}
<p class="chapterhead"><img src="images/Contents.png" alt="contents.png"/></p>