Can't change my header image

I’m using the Twenty Twelve theme and recently bought my own domain. I just have the free wordpress account.

I’m trying to upload a new image for my header. When I upload it, it appears for me but when I log out and look at it as a regular person, it still shows my old image. I know I’m not supposed to change my CSS around but I can’t find any other options in the forums to change my image. I wonder if there’s another way.

My site is http://www.ladydickson.com. The image I’m trying to change is the one with the mountains in the background.

Thanks!