You have a lot of white space around the image in cornbg-l.gif. That's why you see the little corner of the image. You need to crop that to just the image. (Same with cornbg-r.gif, most likely.)

Also, you do need to uncomment the display: block;
Code:
    /*display: block;*/ /*this might be needed to make the span fill the heading*/
You need to change centerboxhead-new.gif to just the text on a transparent background, so you can see the cornbg images behind it.