find image location for css to use ??

[kevin_sauerwald]

ok I have some code that shakes my image.. but its using CSS that gives it a fixed starting point.. and then I have some js that then moves
it around..

/*<![CDATA[*/
body {
     background:#ffffff;
 }
#Logo1{
     position:absolute;
     left:300px;
     top:10px;
     background:#ffffff;
     //border:double 8px #000000;
 }

/*//]]>*/

function vibrate() {

var lleft,LPos,Done;
Done=false;
lleft=310;
// this will set the offset
xx=xx+1;
switch (xx) {
	case (10) :
		lleft=310;
		break;
	case (20) :
		lleft=320;
		break;
	case (30) :
		lleft=330;
		break;
	case (40) :
		lleft=340;
		break;	
	case (50):
		
		Done=true;
		var d2 = new Date;
		var n = d2.getTime();
		document.getElementById("time2").innerHTML = n;
		break;

	default:
		//lleft=300;
	}

LPos=lleft.toString()+"px";


if(document.getElementById("Logo1").style.left==LPos) {
     document.getElementById("Logo1").style.left="300px";
} else {
     document.getElementById("Logo1").style.left=LPos;
}

	if (!Done){
	
		setTimeout('vibrate()',50);
	}
}

its ugly.. but it works.. so this is all fine and dandy.. but it always starts it at left position 300.. I now want to be able to have it
be center screen for anyone.. and then use that as a variable into my js code to know where to return to during its shaking..

any ideas of how to get the center position of my image and then use it with the css position ? I want to have a variable
control this left:300px;
but can you do that in css ? I'm new to css...