I have the same problem, but will have 2 images for 20+ pages. So to use image1 for some, and image2 for others, would this would be the code?
<?php if (is_page('8')('10')('12')) { ?><br />
<div id="header_author"><br />
<?php } ?><br />
<?php if (is_page('11'){'13')('15')) { ?><br />
<div id="header"><br />
<?php } ?><br />
</div>