changing header image in child theme

Hi,
i have created a child theme for original twentyten theme to make changes without affecting the original theme files.
in my child theme i have created style.css file and made changes that change the appearance of my pages. these all work ok.
now i want to change the picture in headers of my pages (change the size of the picture and another file). these definitions are in functions.php. so i created a new functions.php file in the child theme folder and put the changes i want to make in it, but nothing happens. what is wrong?
Thanks.