Alt Tag in Header of TwentyTen Not Appearing

I added text between the alt tag double quotes in the header.php file of the TwentyTen theme but the alt text will not show up in any of my browsers. I’m completely stumped. I didn’t change anything else in the code. What do I need to do to get the alt text to show up on mouseover?

It looks like this:

<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="Photo donated by Robert J. Hurt Landscape Photography" />

Any ideas? Is the alt tag text too long? Thanks for any help you can provide.