Indeed I can see on this page
that the top image is heavily compressed and has a low resolution.
Is your current template or blog framework handling the image conversion itself?
Otherwise I would suggest manually exporting all your source images as WebP images before inserting them with your framework or template.
If that does not work out, you might consider converting the images afterwards.
If you do not have access to the published contents, you might have to change the template or framework for a WebP-ready one.