You can use the WordPress Header feature (under the Appearance admin menu “Header”) or you can use a widget in the Showcase widget position by inserting your image into a text widget. For this option, you would just use html code for an image:
To only show this on the front page, you can use what I’ve been using since Encounters and Celestial Reloaded, which is a plugin called Dynamic Widgets. This lets you show widgets on select pages only.
However, if it’s just a single image, you should be able to just use the WordPress Header option instead.
If you have a slideshow for widgetkit setup, grab the shortcode (get that from the widgetkit slideshow admin) it has and add it to the Advanced text widget’s content area, then set the widget to show only on the front page. Don’t forget to disable the title with the widget’s setting to suppress it.