I finally got sick of bug 2815 (search results should show thumbnails), and added thumbnail display for image results in MediaWiki's core search results display.
Rather than try to port it to the LuceneSearch plugin, I've started rolling out the MWSearch plugin on http://test.wikipedia.org and http://commons.wikimedia.org -- this uses MediaWiki's core search front-end together with our custom Lucene back-end.
This is definitely an improvement for Commons, where search results are now beginning to be actually useful. ;)
But there are definitely some UI elements we should improve on the core interface before rolling it out on other sites -- pretty it up a bit, include the search relevance results from Lucene, etc. Possibly additional thumbnails for pages that _contain_ images but are not themselves image pages.
Some of this will need to be changed in Special:Search; some may need some tweaks to the SearchEngine base class and the MWSearch extension.
-- brion vibber (brion @ wikimedia.org)