custom header

Hi,
as my header for my blog I want to use a 728x 90 leaderboard which I have put my logo on….

here is the CSS style for my theme….
problem is i dont know CSS (im a novice) – how do i need to change it so my picture will be displayed? I have replaced the .jpg with the jpg file I am using, but i think the problem is the SIZE of the file… can anyone help show me where I need to change the SIZE on the coding here?

Thanks!!

/*HEADER*******************************************************************************************************************/
#header{background:transparent url(images/header_title.jpg) no-repeat 0px 0px;height:70px;}
#header h1{font-size:25px;}
#header_l h1 a {
display:block;height:60px;
text-indent:-9999px;width:400px;float:left;}
#header_l {float:left;height:52px;}
#header_r {float:right;padding:15px 10px;text-align:right;width:207px;color:#CCCCCC;}
#header_r a{color:#CCCCCC;}
#header_r a:hover{color:#0099CC;,text-decoration:underline;}
#asides a:hover{color:#333333;text-decoration:underline;}
#header_r span.rss,#header_r span.email{
margin-left:5px;}