There is no FTP access to our WordPress.com blogs and we cannot upload third pary themes. The only themes we can use on a WordPress.com blog are found here . Dashboard > Appearance > Themes
The can also be viewed here Themes Showcase there are feature filters in the search box, decriptions of the themes and live demo sites. Here’s the filter for themes with custom headers. Appearance > Header is where to upload the images after you have correctly sized them ti fit the dimensions of the theme you use.