{Theme: Yoko] Can't replace Gateway theme stock header

I created a new blog at day2asia.wordpress.com using the theme “Gateway.” Ever since, when I try to change the theme and/or the header image in any theme, the stock header image attached to the “Gateway” theme remains on my blog. When I upload a new image, it says that the new image is the “current” image, but the header image on the site still does not change. How can I fix this? Appreciate any help!