I never had luck with any browser extensions that claim to do just that.
I run a console app that relies on AgentRansack to find unused css classes in the project directory (i.e. declared but not referenced in ascx/js etc.) - then it merges the results into a usage report. I'm assuming this could be fairly easily adjusted to look for image srcs etc., as long as the project directory has unused pages removed.
Is there demand for a properly distributed tool like that?