Here is how to change the logo that appears in Zenoss to a custom logo of your choosing:
Go to http://yourzenoss:8080/zport/portal_skins/EnterpriseSkin/manage
Click on zenterprise.css
and then its Customize button
Find zent-img/zenoss-logo-enterprise.png
in the stylesheet and change it to zenoss-logo-enterprise.png
Save the Changes.
Go to http://yourzenoss:8080/zport/portal_skins/EnterpriseSkin/zent-img/manage
Click on zenoss-logo-enterprise.png
and then its Customize button
Upload your replacement image. It should be 318x35 pixels in size.