In a Job Overview the Icons of the disk usage plugins are missing. They 'plugin' icon is shown, but not the folder Images (See the attached screenshot).

I think (without having tested it), its a bug in the jelly code:
https://github.com/jenkinsci/disk-usage-plugin/blob/master/src/main/resources/hudson/plugins/disk_usage/ProjectDiskUsageAction/jobMain.jelly

There you see:

• working: <t:summary icon="/plugin/disk-usage/icons/diskusage48.png">
• not working: <td><img src="${resURL}/plugin/disk-usage/icons/directory16.png" ...