Investigate using WebP for images

[sedders123]

This can help improve page load times on slower connections. Think that support is wide enough we can do this now.

[RealOrangeOne]

Support isn't perfect, but it's not bad.

src: https://caniuse.com/webp

[raccube]

Support doesn't matter. We can provide multiple formats (in most places)

[PeterJCLaw]

I'm not sure it's practical for us to provide multiple image formats, at least at the moment, so support does matter.

At some point we might be able to either proxy the images (https://imgproxy.net/ is a thing) or if we move away from the GitHub Pages default post-process our sites to convert before upload (do tools for that exist?), but I think we're a way away from that being possible.

[raccube]

My point was the <picture> element exists and lets you use a <img> fallback.

[raccube]

Only thing is that we can't have multiple formats for a background-image