help:image source
Sourcing images is important on Danbooru, for both archival purposes and for crediting the artist, and is a mandatory process.
When uploading from another site, such as from Pixiv, Twitter, etc, you can just paste the link onto the upload page and Danbooru will automatically set the correct source. Danbooru supports uploading from most art sites out there, so you should almost never need to save the file to disk before uploading it. See here for a list of supported sites.
If you must upload a file from your computer (for example, because you can't find the original source, or it was deleted), first reverse-search the file using Saucenao, Ascii2D, or Google Image Search, and upload from the right source instead if you are able to find it.
If you are uploading something you commissioned, scanned, or drew yourself, tag your post with commissioner upload, self-scan, or self-upload. Leave the source as the original filename.
If you are uploading a game asset or CG that you extracted or datamined yourself, tag your post with self-datamine. If you're instead uploading it from a wiki or another similar source, put the original link in the source field.