[Simon]

Make sure you use good alt text in your img tags.

Also, if you're comfortable with CSS, you could apply things like visibility, absolute positioning, and z-index values to your first H1 tag and header graphic to have your cake and eat it too. In other words you can position the H1 text (invisibly or not) behind your header graphic, so the graphic is all the surfer sees but the H1 text is still right there at the top of the page.

But mostly I agree with Danger Dave about making sure you know why you want to spend time making header graphics.