Confused about how to change header

I read the documentation on the header, and looked in header.php and the stylesheet.css file, but I still can’t figure out how to do something as simple as:

(1) Making the default header shorter – the default is WAY tall I think or

(2) Replacing the default header with an image.

In stylesheet.css I tried changing the height from the default 192 to just 92, but didn’t see anything change.

Any suggestions would be appreciated. Thanks!

doug