Return Chuck to background image on newer Jenkins.

This is to fix the issue we saw as well with Chuck not appearing in recent Jenkins.  Referenced in this bug:

https://issues.jenkins-ci.org/browse/JENKINS-25084

'main-table' does not exist in newer Jenkins, but I kept a check in for it for backwards compatibility.  Love this plugin by the way, it's my been my first install after Hudson/Jenkins for many years.