Header not working

Can someone please help me, i am using the twenty eleven theme and i am trying to change my header. normally i can change it with in the wordpress itself. its not letting me change it in wordpress. i uploaded them and i can’t see anything. it shows something is there on the website just not the logo i’m looking for. I also put images them in my ftp file in godaddy and still nothing comes up. When i change the theme i can get the header to work. i’m not understanding why it won’t work for this theme? i have unactivated my plugins. Should i reload wordpress or just that theme maybe? is there something else i can do without having to find a whole new theme or can i just put it on the code itself. i’m not sure how to go about that. Does my hearder logo need to be in a certain format like jp or png file?
my website is omginc.us
thank you for any help you can give me
Mak