Display thumbnails of all marked files, in `image-dired-thumbnail-buffer'.
If a thumbnail image does not exist for a file, it is created on the
fly. With prefix argument ARG, display only thumbnail for file at
point (this is useful if you have marked some files but want to show
another one).
Recommended usage is to split the current frame horizontally so that
you have the dired buffer in the left window and the
`image-dired-thumbnail-buffer' buffer in the right window.
With optional argument APPEND, append thumbnail to thumbnail buffer
instead of erasing it first.
Optional argument DO-NOT-POP controls if `pop-to-buffer' should be
used or not. If non-nil, use `display-buffer' instead of
`pop-to-buffer'. This is used from functions like
`image-dired-next-line-and-display' and
`image-dired-previous-line-and-display' where we do not want the
thumbnail buffer to be selected.
(fn &optional ARG APPEND DO-NOT-POP)