This seems to be a static website managed with Jekyll. I've never used it, but I'd be surprised if there isn't a way to automatically convert images to different formats.

I'm assuming it would take a bit longer to deploy the site and having 2 or 3 different versions of the same image would use more space, but it shouldn't be hard that hard to use AVIF, even if Pixelmator doesn't support it at the moment. It would also ensure that those with older browsers can still view the image, something that won't happen if we only have AVIF or WebP.

They shouldn't be manually exporting web formats