Go to the Market Theme settings area.

There is a section to upload a header graphic. The title is "Upload a Custom Banner Image:"

Just click the "Upload" toggle button and browse your computer for the banner header image you desire.

When you find the image you want to use, just double click it and the click on the "Upload Image" button.

Then, just click the "Activate Changes" button at the top or bottom of the page.

Just a heads up… Market Theme will stretch the banner image width to match the width of your website (if it's not already the same). So, if you don't want it getting stretched — edit the image on your computer so that it's the same width before uploading.

Yes you are. Look under the "Market Theme" tab then settings.