It all seems to depend on what I set for the $wgSharedUploadDirectory in
the
LocalSettings file of the language wikis. But I might be wrong. I set: $wgSharedUploadDirectory = 'http://media.jesus-wiki.org/w/images';
And exactly that's the problem. You have to put a local filesystem path in there, so that convert can actually get the file, convert it and write a converted version back.
HTH chris