20 eleven: reducing the height of the header image

Hello everybody
I am trying to create my photo website with “20/11” but I need to reduce the height of the header image.
indeed, i dont like when I connect to the home page that the header image cover half screen
i want to give more space to the images of my gallery
there is a simple way to do it, or do i have to modify the css?
Can i customize from the menu of the theme?

another question is: is this theme specific for photoblogging?
Thanks and sorry for simple questions, i am pratically new of WP world.
If needed, i can post the website address but at the moment is in mantainance mode.
Paolo