For SEO purposes, you can have an ALT tag on the image. The huffington post does just that:
<h1>
<a href="">
<img src="https://s.huffpost.com/images/v/logos/v4/homepage.gif?v8" alt="The Huffington Post" />
<!--
-->
<br/>
<img src="https://s.huffpost.com/images/v/logos/v4/tagline.gif" width="460" height="9" class="margin_top_5" alt="The Huffington Post" />
<!--
--> </a>
</h1>