Hi!

This is a very helpful and straightforward article, and the first one that advise to use nginx (instead of apache) and wordpress together.

Regarding to the caveat...

You may enable convert_jpeg_to_webp filter. To avoid having problems with varnish cache you should add the next lines to vcl_fetch:

if ( req.http.X-Accept ) {
hash_data (req.http.X-Accept);
}

And, in vcl_recv:

if ( req.http.Accept ~ ".*image/webp.*" ) {
set req.http.X-Accept = "GoogleImageFormat";
}
if ( req.http.Accept !~ ".*image/webp.*" ) {
set req.http.X-Accept = "other";
}

With that code, varnish will make a difference between Chrome a firefox browsers.