DouKaku?

(use graphics.gd)
(use rfc.http)
(use srfi-42)

(define (get-and-invert-jpeg host path output)
  (receive (status hdrs body) (http-get host path)
    (unless (equal? status "200") (error "http-get failed:" status))
    (let1 img (read-gd-image (open-input-string body) 'jpg)
      (do-ec (: x (gd-image-sx img)) (: y (gd-image-sy img))
             (let1 pixel (gd-image-get-pixel img x y)
               (gd-image-set-pixel img x y
                                   (gd-true-color (- 255 (gd-image-red img pixel))
                                                  (- 255 (gd-image-green img pixel))
                                                  (- 255 (gd-image-blue img pixel))))))
      (call-with-output-file output (cut write-as img 'jpg <>)))))