image-dired-cmd-create-thumbnail-options

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

"%p -size %wx%h \"%f\" -resize \"%wx%h>\" -strip jpeg:\"%t\""

Documentation:
Format of command used to create thumbnail image.
Available options are %p which is replaced by
`image-dired-cmd-create-thumbnail-program', %w which is replaced by
`image-dired-thumb-width', %h which is replaced by `image-dired-thumb-height',
%f which is replaced by the file name of the original image and %t
which is replaced by the file name of the thumbnail file.

You can customize this variable.