1. When you start changing themes you should make a child theme folder. if your theme folder is /httpdocs/wp-content/themes/twentyten then make the child folder /httpdocs/wp-content/themes/twentyten-child. you then need the subfolders /images/header. You will need to be familiar with WordPress child themes. If you replace the orginal folders image, you may lose what you’ve done if there is a theme or other for of WordPress update.

2) Give your picture the same name as one of the twenty ten default picture. Keep the default size of 940 pixels by 198 pixels).

3) Upload your new image into /httpdocs/wp-content/themes/twentyten-child/images/headers

4) The header should be automatically updated if in the dashboard you have chosen the child theme already and are using the default WordPress image.