Prepare img size max 400px width X max 70px height
open style css, find line 41
#header{background:transparent url(images/sl.jpg) 0 12px no-repeat;width:100%;height:90px;display:block;margin:5px auto 0;padding:0;}
change img name to your image name
find line 42
.head{background:transparent;height:65px;display:inline-block;float:left;margin:0;padding:10px 0 0 40px;}
change to
.head{background:transparent;height:65px;display:inline-block;float:left;margin:0;padding:10px 0 0 0;}
find line 43
.head h1 a,.head h1{text-decoration:none;outline:none;height:65px;display:block;color:#333;}
change to
.head h1 a,.head h1{outline:none;width:400px;height:65px;display:block;text-indent:-99999px;}
this way you have a semantically correct replacement, which works also for screen readers.