adding another logo that is called when my custom header.php is used

I am working on a site that is merging two sites into one. So they want to be able to have a different logo display in the header when navigating a certain section of the website.

I created a custom template and then in that template call a custom header.php page. But now I do not know what to do from there to have it call a different logo image?

Any help would be appreciated. Let me know if you need more information on the issue. The site is being built locally at the moment so I have no link.

Thanks