Это должно сработать
var express = require("express");
var logger = express.logger();
var app = express.createServer();
var conditionalLogger = function (req, res, next) {
if (!(/\.(png|jpg|gif|jpeg)$/i).test(req.path)) {
logger(req, res, next);
} else {
next();
}
}
app.use(conditionalLogger);
app.use(express.static("./public"));
app.listen(3456);