Also, I double-checked my version of image-dired using list-packages, and it was listed as 0.4.11. I believe the source is here

I want to "mark" the images with image-dired-mark-thumb-original-file, which marks the associated file in the dired buffer, and then rename/move the marked files as usual in the dired buffer.

(define-key image-dired-thumbnail-mode-map "l" 'image-dired-forward-image)
(define-key image-dired-thumbnail-mode-map "h" 'image-dired-backward-image)
(define-key image-dired-thumbnail-mode-map "u" 'image-dired-unmark-thumb-original-file)
(define-key image-dired-thumbnail-mode-map "m" 'image-dired-mark-thumb-original-file)

But image-dired-thumbnail-mode-map does not seem to be defined. Restarting Emacs gives me this error: emacs symbols value as variable is void: image-dired-thumbnail-mode-map

Do I need to instantiate image-dired-thumbnail-mode-map somehow? My code above was influenced by this documentation as well, so I don't understand why I am getting an error.

I want to move multiple images at a time, so I am "marking" the thumbnailed images with image-dired-mark-thumb-original-file, which marks the associated file in the dired buffer. After this, I can move all of the marked files as usual using dired.

(add-hook 'image-dired-thumbnail-mode-hook
      (lambda ()
            (define-key image-dired-thumbnail-mode-map "l" 'image-dired-forward-image)
            (define-key image-dired-thumbnail-mode-map "h" 'image-dired-backward-image)
            (define-key image-dired-thumbnail-mode-map "u" 'image-dired-unmark-thumb-original-file)
            (define-key image-dired-thumbnail-mode-map "m" 'image-dired-mark-thumb-original-file)
            )
          )

I also tried substituting "l" with (kbd "l"), but no success. I am still getting the same issue, where my mode help menu lists the key bindings as currently shadowed by another mode. C-h m shows the same conflicts:

h               image-dired-backward-image
  (that binding is currently shadowed by another mode)
l               image-dired-forward-image
  (that binding is currently shadowed by another mode)

Any suggestions?