It is going to take some work to make it fit in that theme.
1. upload your image to your theme’s image directory.
2. goto your wordpress dashboard=>appearance=>editor
It should have style.css open automatically. Look for this code:
.header
{
width: 940px;
margin: 0 auto;
height: 155px;
}
Add these lines to make it look like this:
.header
{
width: 940px;
margin: 0 auto;
height: 155px;
background-image: url("images/YOUR_FILE");
background-repeat: no-repeat;
}
Don’t forget the extension on your picture file .jpg or .png, etc.
From there, we’ll have to mess with your other elements to make it look right.