ok here's my css, I have been looking all over the blasted internet for how to center something in an absolute position but nothing is working..

Code:
<div style="position:absolute; width:100%; height:341px;">
      <img src="pics/clinic1.jpg" width="910" id="clinic1" style="position:absolute; top:0; margin-left:auto; margin-right:auto;"/>
      <img src="pics/clinic2.jpg" width="910" id="clinic2" style="position:absolute; top:0; margin-left:auto; margin-right:auto;"/>
        </div>
I have a script that fades in and out between the images but I needed to know how to put them both in the same spot on the page (centered) so that the effect works right... right now they just appear in the same spot but I cant get them to center. Help??