image-dired-cmd-create-standard-thumbnail-command

image-dired-cmd-create-standard-thumbnail-command is a variable defined in `image-dired.el'.
Its value is

"convert -size %wx%h \"%f\" -set \"Thumb::MTime\" \"%m\" -set \"Thumb::URI\" \"file://%f\" -set \"Description\" \"Thumbnail of file://%f\" -set \"Software\" \"GNU Emacs 25.0.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.6)\n of 2014-12-27 on Gandalf-Linux\" -thumbnail \"%wx%h>\" png:\"%t\""

• This variable may be risky if used as a file-local variable.

Documentation:
Command to create thumbnails according to the Thumbnail Managing Standard.

You can customize this variable.