…is not possible without the CSS upgrade. But you can display an image below the header. Create a 958px wide image, upload it via Media > Add New, copy its URL. Then go to Appearance > Titan Options, click “Homepage Notice”, paste the following code in the “Custom notice” box, click “Save Changes”:
Minimum image height: 60px. Suggested maximum height: 120px. Looks better with no top navigation (Appearance > Titan Options: click “Navigation”, tick all three options, click “Save Changes”).
Update Nov. 2011: Workaround no longer needed – Titan now supports a regular header image (960×180 pixels, uploaded in Appearance > Header).