hi ,
I have your answered this question posted many times , try a simple Search next time ..
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css/index.css Find:::
/* BANNER / LOGOS */
#logo {
position: absolute;
top: 30px;
left: 20px;
display: block;
width: 256px!important; <===Your image Width===>
height: 61px!important; <===Your image Heigth===>
cursor: pointer;
background: url(../images/img/logo.png) no-repeat!important;
}