chore: optmise images

When [analysing codebar.io with pagespeed][0], it is clear that the
majority of images can benefit from optimisation.

This commit updates all images in `app/assets/images/` with optimised
versions, that have been optimised using [ImageOptim][1].

[0]: https://pagespeed.web.dev/analysis/https-codebar-io/5nuwo09wma?form_factor=desktop
[1]: https://imageoptim.com/mac

Author: mroderick