func blackHandler(w http.ResponseWriter, r *http.Request) {

    key := "black"
    e := `"` + key + `"`
    w.Header().Set("Etag", e)
    w.Header().Set("Cache-Control", "max-age=2592000") // 30 days

    if match := r.Header.Get("If-None-Match"); match != "" {
        if strings.Contains(match, e) {
            w.WriteHeader(http.StatusNotModified)
            return
        }
    }

    m := image.NewRGBA(image.Rect(0, 0, 240, 240))
    black := color.RGBA{0, 0, 0, 255}
    draw.Draw(m, m.Bounds(), &image.Uniform{black}, image.ZP, draw.Src)

    var img image.Image = m
    writeImage(w, &img)
}