image-load-path

image-load-path is a variable defined in `image.el'.
Its value is

("/usr/local/share/emacs/25.0.50/etc/images/" data-directory load-path)

Documentation:
List of locations in which to search for image files.
If an element is a string, it defines a directory to search.
If an element is a variable symbol whose value is a string, that
value defines a directory to search.
If an element is a variable symbol whose value is a list, the
value is used as a list of directories to search.

Subdirectories are not automatically included in the search.

You can customize this variable.