image-dired-cmd-rotate-thumbnail-options

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

"%p -rotate %d \"%t\""

Documentation:
Format of command used to rotate thumbnail image.
Available options are %p which is replaced by
`image-dired-cmd-rotate-thumbnail-program', %d which is replaced by the
number of (positive) degrees to rotate the image, normally 90 or 270
(for 90 degrees right and left), %t which is replaced by the file name
of the thumbnail file.

You can customize this variable.