What you'll want to do is add CSS to replace the masthead title with your image.
First, upload your image to the media library in your blog dashboard and copy the image URL.
Then you can add something like this to do an image replacement in the DePo Masthead theme:
#container {
margin-top: -70px;
}
h1.name {
height: 145px;
}
h1.name a {
background: white url(http://s.wordpress.org/about/images/wordpress-logo-notext-bg.png) center -10px no-repeat;
display: block;
width: 176px;
height: 125px;
margin: 0 auto;
}
h1.name a span {
display: block;
text-align: left;
text-indent: -9999px;
background: none;
}
I used a WordPress logo as an example because you didn't provide an image link.
Note that the example above is specific for the image I used. You will have to change around several of the size settings and the url() value in order to change this to an image from your own media library.