Adds the background photographs that the `list_colours` and `list_shapes` generators draw over, so every task in this repository can be regenerated from the repository alone.
- `background_image/bg_1.png` … `bg_10.png` at the repository root: the ten photographs both generators composite their shapes onto. One shared copy, since both scripts use the same set.
- `README.md`: the layout and the "Regenerating the data" section point at the new folder; the licence section is reduced to the CC BY 4.0 line; a changelog entry is added.
Both generators were run against exactly these files and produce `data.json` records with the same fields and types as the released data, and images of the same mode and dimensions.
