An image that was posted on Danbooru by its own creator.
If the image was posted directly to Danbooru without being posted anywhere else first, it will also have a Non-Web Source.
Self-uploads are allowed on Danbooru, but be aware that Danbooru is a moderated site and uploads may be deleted if they are Off-Topic or aren't of sufficient quality. See Help:Upload and About:Mod Queue for details. Compare Self-Upload Is:Active versus Self-Upload Is:Deleted to get a sense of the expected quality.