The individual elements of the Lister (e.g. the file display background, the folder tree, etc.) support images as well as colors.
Note that when you edit images on this page you're only editing the images in the current set ("dark" or "light") - use the options on the Themes page to select which color and image set is in use.
You can use the browse button in the image picker to select an image. The dropdown shows you images that are in use elsewhere in case you want to use the same one in multiple places.
When an image is selected, you can choose the alignment:
The Shared option lets you share an image across multiple elements. Shared images are aligned with the whole Lister rather than the element they're assigned to - if two or more elements share an image, the image in one will join seamlessly with the image in another.
You can also specify an opacity for the image if desired.