Large Image in <div> from Thumb Using DOM in a WordPress Theme

I hope I’ve found the right place to ask this question. I’m a bit of a DOM newbie.

I want to show the fullsize image in an absolutely positioned div whenever someone mouses over a hyperlinked image in a specified <div>. I cannot add any classes or ids to the anchor or image elements, so getElementById is not of much use.

The HTML snippet that I need to trigger the event via the DOM is:

<div id="ComplexItem">
*Trigger here >> <a href="URI"><img src="URI" /></a>
</div>

I should be able to handle the code that does the actual work. It’s figuring out how to trigger the Javascript via the DOM that troubles me.