size of header picture

I found it ?? functions.php

Could you tell me please where is that file?

you open file funtions.php in themes then add code
add_theme_support( ‘post-thumbnails’ );

Once you have activated support for post thumbnails, you can use the function add size image registration with add_image_size function (). The function is used add_image_size this: add_image_size (‘name-of-size’, width, height, crop mode);
Eg:
add_image_size( ‘sidebar-thumb’, 120, 120, true ); // Hard Crop Mode
add_image_size( ‘homepage-thumb’, 220, 180 ); // Soft Crop Mode
add_image_size( ‘singlepost-thumb’, 590, 9999 ); // Unlimited Height Mode

<font color=”red”>Mr:Bao</font></b