I would like to know if the way I am handling the H1 tag on my site is correct. This is a wordpress site with Thesis theme.

Because I use a header logo rather just just printing the title and tagline on the page, I end up with the following generated code, according to viewing the source:

<div id="header">
<h1 id="logo"><a href="http://www.mystie.com">Free online college courses by mysite.com</a></h1>
</div>

And this is my CSS for #logo:

.custom #logo {background-image: url('/images/logo.jpg'); background-repeat: no-repeat; width: 600px; height: 62px; text-indent: -9999px; }
.custom #logo a {width: 600px; height: 62px; display: block; outline: none;}

So, this is placing my site title or tagline (depending upon my settings) wording in the H1 tag. But it doesnn't display on-page since apparently the background image blocks it.

So, my question is, how do search engines treat this H1 tag as done above? Do they treat the text in "Free online college courses by MySite.com" with the power of an H1 tag that has its text displayed (without the background-image)? Or is it of less value somehow since the text is not actually displayed?

If it's not of less value, than this current method works for me. If, however, I actually were to display that text in an H1 tag with without the background image, and the search engines liked that better, I would find a place to actually display the H1 tag's wording.