How to customize header in Twenty Eleven

I am VERY new to WordPress & would appreciate any help on how to change alignment, size & add photo for header. This is my site photomomentsforlife.com/wordpress. Right now the header is text that doesnt match in size & I would rather it be a logo image centered in the middle at the top of the page. I have made a child theme & it is activated with a style.css but it seems like no matter what codes I try it doesn’t change a thing. Please list specific codes that I should add to my style.css. Thanks for your help!