From the images

generate these:

Now moving the grid on the combined image will result in images like these:
;; -*- mode: lisp; syntax: common-lisp -*-
;; Peter Salvi, 2006.
;; Animation Maker for ż÷

;; File I/O

(defun skip-whitespace-and-read-number (stream)
  "Reads a decimal number from a stream that may have whitespace before it,
and _must_ have whitespace after it.
WARNING: This function also eats the first whitespace after the number."
  (let ((next (read-byte stream))
        (whitespace-list (mapcar #'char-code '(#\Space #\Tab #\Newline #\#)))
        (result 0))
    (loop while (member next whitespace-list) do
          (when (= next (char-code #\#))
            (loop until (= (read-byte stream) (char-code #\Newline))))
          (setf next (read-byte stream)))
    (loop until (member next whitespace-list) do
          (setf result (+ (* result 10) (- next 48)))
          (setf next (read-byte stream)))

(defun read-pbm (stream)
  "Reads a PBM stream, and returns the contents as a two-dimensional array."
  (unless (and (= (read-byte stream) (char-code #\P))
               (= (read-byte stream) (char-code #\4)))
    (error "Not a plain PBM stream"))
  (let* ((width (skip-whitespace-and-read-number stream))
         (height (skip-whitespace-and-read-number stream))
         (result (make-array (list width height))))
    (dotimes (j height)
      (dotimes (i (ceiling width 8))
        (let ((c (read-byte stream)))
          (dotimes (k 8)
            (when (< (+ (* i 8) k) width)
              (setf (aref result (+ (* i 8) k) j) (logbitp (- 7 k) c)))))))

(defun write-raw-pbm (stream array)
  "Writes the contents of ARRAY to a stream in raw (ASCII) PBM format."
  (multiple-value-bind (sec min hrs day month year)
    (format stream "P1~%~
                    # Generated by Animation Maker @ ~d-~d-~d ~d:~d:~d~%~
                    ~d ~d~%"
            year month day hrs min sec
            (array-dimension array 0) (array-dimension array 1)))
  (dotimes (j (array-dimension array 1))
    (dotimes (i (array-dimension array 0))
      (format stream "~:[0~;1~]" (aref array i j)))
    (format stream "~%")))

(defun load-pbm-file (filename)
  "Convenience function for loading PBM files by filename."
  (with-open-file (stream filename
                          :direction :input :element-type '(unsigned-byte 8))
    (read-pbm stream)))

;; Generating images

(defun generate-key (width height n)
  "A pattern of vertical bars, with every nth being white."
  (let ((result (make-array (list width height) :initial-element t)))
    (dotimes (i (1+ (floor width n)))
      (dotimes (j height)
        (setf (aref result (* i n) j) nil)))

(defun combine-pictures (lst)
  "Combines N pictures by taking the Ith column from the (I mod N)th picture."
  (let ((n (length lst))
        (width (array-dimension (first lst) 0))
        (height (array-dimension (first lst) 1))
        (result (make-array (array-dimensions (first lst)))))
    (dotimes (i width)
      (dotimes (j height)
        (setf (aref result i j) (aref (nth (mod i n) lst) i j))))

(defun animate (output key &rest pictures)
  "Convenience function for generating the `animated' and `key' image from
multiple original images."
  (let ((n (length pictures))
        (combined (combine-pictures (mapcar #'load-pbm-file pictures))))
    (with-open-file (stream output :direction :output)
      (write-raw-pbm stream combined))
    (with-open-file (stream key :direction :output)
      (write-raw-pbm stream (generate-key (array-dimension combined 0)
                                          (array-dimension combined 1)

;; Test

;; Horse images from
(animate "horse-animation.pbm" "horse-key.pbm"
         "horse1.pbm" "horse2.pbm" "horse3.pbm" "horse4.pbm")