PNGs are not opimised (performance issue)

[Google Lighthouse](https://developers.google.com/web/tools/lighthouse/) identified that we ship PNGs with a larger file size than needed:

One script which may help is:
```bash
# Warning: If the optimization fails or the file can't be optimized, the file ends up as a 0 byte file
find . -name '*.png' -exec mv {} {}.old \; -exec pngcrush {}.old {} \; -exec rm {}.old \;
```

Start by inventorying the repository's PNG assets and reviewing the linked Lighthouse finding to identify which files are oversized. Use the issue's pngcrush approach cautiously with backups, then verify that the optimized files are smaller and the website remains intact.

Written by the indexing model from the issue text.