image-dired-thumbnail-storage

image-dired-thumbnail-storage is a variable defined in `image-dired.el'.
Its value is

use-image-dired-dir

Documentation:
How to store image-dired's thumbnail files.
Image-Dired can store thumbnail files in one of two ways and this is
controlled by this variable. "Use image-dired dir" means that the
thumbnails are stored in a central directory. "Per directory"
means that each thumbnail is stored in a subdirectory called
".image-dired" in the same directory where the image file is.
"Thumbnail Managing Standard" means that the thumbnails are
stored and generated according to the Thumbnail Managing Standard
that allows sharing of thumbnails across different programs.

You can customize this variable.