Replace 2010 Theme Site Title with a Graphic

The twenty ten title is text if you wanted to add in a logo graphic you would do this by creating a new header with the logo overlayed and uploading this.

Then you would hide the text title and description by adding a couple of lines at the end of the style.css file.

/* hide site title */
#site-title {
    display:none;
}
/* hide site description */
#site-description {
    display:none;
}

HTH

David

The header height is set in the functions.php file, on line 111.

define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 198 ) );

Polite Suggestion
If you are doing changes to the twenty ten theme then consider creating a child theme and develop and test in a local environment.

There you would not change the default theme but add changes in the child themes functions.php

/** Tell WordPress to run child_theme_setup() when the 'after_setup_theme' hook is run. */
add_action( 'after_setup_theme', 'child_theme_setup' );

if ( !function_exists( 'child_theme_setup' ) ):
function child_theme_setup() {
	define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 320 ) );
}
endif;

If you are new to child themes here is a download for a twenty ten test theme, it has the starting files including the after_theme_setup function above without the code.

Just open the style.css and change the theme name uri etc:

HTH

David