There are quite a few cases when you'll want to use an image for a headline, section header and the like, but want to keep a text 'backup' so that search engines and screen readers can understand what the headline says. You can do this with a CSS image replacement technique.
Basically, you create a text header, and then use a CSS class to text-indent
the text way off screen, and replace it with an image:
<h1 class="myHeadline"> Really Important Headline </h1>
h1. myHeadline { width: 500px; height: 40px; background: url("images/really-important-headline.jpg"); text-indent: -9999px; }
N.B.: This image is called the 'Phark' method, and is the most popular method in use today. However, it is far from the only one: CSS Tricks has an excellent summary of the top nine.
N.B.: There is also some difference of opinion as to whether this constitutes a 'black hat' SEO technique, since you're hiding text. General consensus is that it is OK to use it occasionally (e.g. a few headlines, your logo, etc.), but don't go overboard.