Basic HTML - Unordered List, how to make it my images vertical?

I have two images and would like to place one on top of another using a list. The current code I'm using places the second image to the right of the first one, not directly below it.

Any ideas? Might be basic but I'm new to programming. Thanks.

<ul>
<li>
<img width="178" height="178" alt="ZE40" src="images/ze40.jpg">
<img width="178" height="40" alt="ZE40" src="images/ze40i.jpg">
</li>
</ul>