OurBigBook
Where the image was taken from, e.g.:
\Image[https://upload.wikimedia.org/wikipedia/commons/6/68/Akha_cropped_hires.JPG]
{title=A couple}
{source=https://en.wikipedia.org/wiki/Human}
which renders as:
Figure 22.
A couple
. Source.
The source is automatically inferred for certain known websites, e.g.:

Ancestors

  1. Image argument
  2. Image
  3. Macro
  4. OurBigBook Markup
  5. OurBigBook Project