Sounds like you are missing the definition in your language files:
PHP Code:
/includes/languages/english/header.php

Line #24 : define('HEADER_LOGO_IMAGE', 'logo.gif'); 
This would also be in your templates and overrides directory if you are customizing the file:
/includes/languages/english/your_template_dir/header.php

NOTE: on DEFINE definitions you cannot just delete them ... they should be set to no value or something else if you are not using them ...

In this case you could set the logo.gif to an image that is a pixel in size if you do not use it ... or edit your header.php to not call that file ...