As we know the Search engines like Google, Yahoo, MSN and Bing are looking for the text of the pages primarily to index them. We know that how to use the header tags <h1>. The <h1> header tags are important because search engines value organized sectioned content.
As the header <h1> is having default properties. But you dont want the big text for your headers on your site. So why dont you use your custom graphics. <h1> are very friendly to search engines, and you dont want to skip your <h1> for <img> tag, It will be loss of search engine friendlyness. so dont do it.
Read More at : http://varadesigns.com/webdesigning-tips/replace-Image-with-header-text.html