You'll probably want to put it in either the #header, #header > div.inner or #canvas > div.inner depending on how you want it to respond to things like browser width and content columns.

Here's an example of how I inserted it simply into the header.
#header {
width: 75%;
background-image: url(https://i.imgur.com/646yBWd.png);
background-size: contain;
}

You can see this live at iconografik

If this is the placement you're looking for you might want to nudge the nav module down depending on the content in the header image.

Alternatively, if you want it to span the full width of the browser window it would have to modify #canvas > div.inner instead.