There is no value set for the height of the image, the 261 value is the height of the header.
The easiest way would be to modify the height of the image with a software like, Photoshop or Gimp or you can modify the index.template.php file to find:
<img src="', $settings['header_logo_url'], '" style="margin: 4px;" alt="', $context['page_title'] != "$context[forum_name] - Index" ? "$context[forum_name] - " : '', $context['page_title'], '" />
and replace it with:
<img src="', $settings['header_logo_url'], '" style="margin: 4px;" height="X" width="Y" alt="', $context['page_title'] != "$context[forum_name] - Index" ? "$context[forum_name] - " : '', $context['page_title'], '" />