Use the images subdirectory in the en-US/ directory to store images. Use ./images/image-name to insert images into a book. The following is an example that inserts the testimage.png image:

<mediaobject>
	<imageobject>
		<imagedata fileref="./images/testimage.png" />
	</imageobject>
	<textobject><phrase>alternate text goes here</phrase></textobject>
</mediaobject>