Thank you guys, here is what is put in my functions.php file:
$custom_header_support = array(
'width' => apply_filters( 'twentyten_header_image_width', 980 ),
'height' => apply_filters( 'twentyten_header_image_height', 222 )
);
add_theme_support( 'custom-header', $custom_header_support );
It works! I wonder if the fact that it is not enclosed by a function can be a problem?
sorry;
<ul>
twentyten</ul>
<li>functions.php</li>
<li>blabla subfolders</li>
<ul>
childoftwentyten</ul>
<li>style.css</li>
<li>functions.php</li>
[Bump deleted per http://codex.wordpress.org/Forum_Welcome#No_Bumping ]
Thanks guys, it is fixed. I just put the right caracter wp-content!
