Pick format from user params

lib/cartodb/controllers/named_static_maps.js

@@ -166,7 +166,7 @@ NamedStaticMapsController.prototype.named = function(req, res) {
                 headers: _.clone(fakeReq.headers),
                 params: _.extend(_.clone(fakeReq.params), {
                     token: layergroupId,
-                    format: 'png'
+                    format: req.params.format
                 })
             };