If you define the style for div.center-image, it's pointless to use <span class="center-image">, or to add a "center-image" class to anything other than a <div>. Moreover, a <span> does not use a text-align property. And you can't have block-level elements (like <div>) inside a <p>.
Try this:
Code:
<div align="center-image">
<img src="images/Stern_title_fmt.jpeg" alt="Stern_title.jpg" />
</div>
(And don't use arbitrary text for the "alt" attribute. It is intended to be used when the image cannot be displayed, for example when a text-to-speech system is being used. If the image is purely decorative, you can have alt="", otherwise use at least something descriptive of what appears in the image.)