image-dired-cmd-pngcrush-program is a variable defined in `image-dired.el'.
Its value is
nil
This variable may be risky if used as a file-local variable.
Documentation:
The file name of the `pngcrush' program.
It optimizes the compression of PNG images. Also it adds PNG textual chunks
with the information required by the Thumbnail Managing Standard.
You can customize this variable.