The only themes we can use on free hosted WordPress.com blogs are those found here > Appearance > Themes and there is no upgrade you can purchase that changes that as there is no upgrade that will provide FTP access and the ability to upload third party themes to a free hosted WordPress.com blog. To do that you must hire a web host and set up your own WordPress.org install.
That's no theme we can use on a free hosted WordPress.com blog with the feature you describe. http://wordpress.com/themes/
There are themes that have featured posts image sliders. http://theme.wordpress.com/themes/features/post-slider/ In most of the themes with featured post sliders, visitors have to click to advance the slider to the next featured post. Some themes have a featured post slider that automatically advances: Debut, Currents, Linen, On Demand, Modularity Lite. (the first 4 are premium themes) and the last one is a free theme. Sight is a free theme with an auto-advancing post slider.