I've started a process of forcing old rendered thumbnails to regenerate. This will gradually replace old rendered images over the next couple days as several issues have been fixed recently:
* Many PNG thumbs defaulted to unnecessarily high bit depth, wasting bandwidth * JPEG thumbs were at a somewhat higher quality than needed, wasting bandwidth * Some PNG and JPEG thumbs were sized a pixel off due to mismatches in how MediaWiki and ImageMagick rounded the resized aspect ratio; these could get resized in the client, leading to extra blurry scaling * Lots of SVG rendering problems fixed by upgrading to librsvg 2.14.0
Since invalidating *all* thumbs brought things to a halt doing nothing but rerendering, I've currently got it invalidating thumbs over a year old, and will be moving up the cutoff date gradually over the next day or two.
-- brion vibber (brion @ pobox.com)