How to changes Zimbra Logon picture

Just put your custom logo .mg files on /opt/zimbra/jetty-6.1.5/webapps/zimbra/skins/_base/logos/LoginBanner.png , don’t forget to changes the files owner to zimbra:zimbra

if you want to edit logo url or page you can edit the skin.properties go to
/opt/zimbra/jetty-6.1.5/webapps/zimbra/skins/_base/base/skin.properties

edit this line :
LogoImgDir = /zimbra/skins/@SkinName@/logos
LogoURL = http://www.yourcompany.com

You can also changes the other modifcation on the login pages on /opt/zimbra/jetty-6.1.5/webapps/zimbra/WEB-INF/classes/messages/ZmMsg.properties