The icon image of FindBugs plugin on job page is broken from FindBugs Plugin 4.7.
The icon attribute of t:summary in summary.jelly uses "${rootURL}", but it is unnecessary.

<t:summary icon="${rootURL}/plugin/findbugs/icons/findbugs-48x48.gif">
${it.result.summary}
->
<t:summary icon="/plugin/findbugs/icons/findbugs-48x48.gif">
${it.result.summary}

See the description of lib/hudson/summary.jelly.

<st:attribute name="icon" use="required">
link to the icon image. relative paths will be resolved against images/48x48,
and absolute paths (that start with '/') will be resolved against the context root of Hudson
</st:attribute>