image-dired-append-when-browsing

image-dired-append-when-browsing is a variable defined in `image-dired.el'.
Its value is

nil

Documentation:
Append thumbnails in thumbnail buffer when browsing.
If non-nil, using `image-dired-next-line-and-display' and
`image-dired-previous-line-and-display' will leave a trail of thumbnail
images in the thumbnail buffer. If you enable this and want to clean
the thumbnail buffer because it is filled with too many thumbnails,
just call `image-dired-display-thumb' to display only the image at point.
This value can be toggled using `image-dired-toggle-append-browsing'.

You can customize this variable.