Edit Oct 25, 2017: Thank you to Andy for the latest solution in the comments and for n3rdly confirming this works on Jenkins 2.86
As much as I adore the butler it’s always fun to customize things. The simplest way I found to change the logo is to use a theme plugin:
You can write a CSS template, say theme.css:
and then place pumpkin.png and theme.css into userContent folder in the home directory.
Once the plugin is installed, you will have access to some extra configuration settings in “Configure System” under “Manage Jenkins” that you will need to update: