Function: image-dired-dired-toggle-marked-thumbs

Toggle thumbnails in front of file names in the dired buffer.
If no marked file could be found, insert or hide thumbnails on the
current line. ARG, if non-nil, specifies the files to use instead
of the marked files. If ARG is an integer, use the next ARG (or
previous -ARG, if ARG<0) files.
(fn &optional ARG)