Formalize figures requirements and guidelines

We have been trying to transition from JPG and PNGs to SVG and generally scalable image formats.

I'd like to formalize this, so we can have clear requirements on the image formats and guidelines on colors/plots scales and image sizes, both for new additions and conversion of existing figures.

No files or tests are named. Start by reviewing the existing JPG and PNG figures and the surrounding documentation, then define requirements for scalable formats, colors, plot scales, and image sizes covering both new figures and conversions. Done means the requirements and guidelines are documented clearly enough to apply consistently.

Written by the indexing model from the issue text.