[Theme: Twenty Eleven] Editing functions_php of child theme of twenty eleven

I am attempting to change the header image of the twenty eleven theme to 1245×200 px. I have found for me, the only way to get this done is to add lines to the child themes functions_php. However, everything I add to the functions_php appears at as text at the top of every page on my website and the intended changes seems to be set. I can’t seem to find anything related to this, but might have missed it. How do I correct this? Thanks in advance.