(or the equivalent) on the affected server(s) would cause the broken images to all be automatically corrected, as needed. Am I wrong? Or are there too many servers, all of which that "quick and simple" command would have to be run on, or something?
MediaWiki's configuration can be slightly tweaked such that all previously cached thumbnails will be regenerated, using what we call the "thumbnail epoch", which is like our other "cache epochs" - but realistically, the system administrators may adopt a mixture of solutions to try and minimise any excessive load spiking due to cache misses or any other funky fun stuff like that.
Presumably though if the size of a thumbnail is 0 bytes, then MediaWiki can safely assume that something went wrong, and that the cache is bad. E.g. Would an "if( filesize( $thumbnail ) === 0 )" or similar test help when determining whether to use the cache or not?
-- All the best, Nick.