image-dired-line-up-method

image-dired-line-up-method is a variable defined in `image-dired.el'.
Its value is

dynamic

Documentation:
Default method for line-up of thumbnails in thumbnail buffer.
Used by `image-dired-display-thumbs' and other functions that needs
to line-up thumbnails. Dynamic means to use the available width of
the window containing the thumbnail buffer, Fixed means to use
`image-dired-thumbs-per-row', Interactive is for asking the user,
and No line-up means that no automatic line-up will be done.

You can customize this variable.