Hosted picture have an ETAG without double quote like:
$ curl -I https://upload.wikimedia.org/wikipedia/commons/thumb/6/69/Flag_of_South_Carolina.svg/23px-Flag_of_South_Carolina.svg.png | grep etag etag: dfb0c6f55f2d606b33afc130f05c7f88
But a few other (static?) images have an etag with double quotes:
$ curl -I https://bm.wikipedia.org/static/images/project-logos/bmwiki.png | grep etag etag: "3e7c-5a6b0464619c2"
It should be coherent, and it seems the norm says it has to have double-quote, see https://developer.mozilla.org/en/docs/Web/HTTP/Headers/ETag
The reason for the discrepancy is that the former image is hosted on Swift, which sends unquoted ETags: https://bugs.launchpad.net/swift/+bug/1099087. The bug is fixed in Swift 2.24.0.