replace img tags for converted images

[Gabriel Kardonski]

when an image is converted to webp, all instances that use that image should be loaded with the picture tag.
https://web.dev/serve-images-webp/#serve-webp-images

For using webp in CSS refer to the link below. perhaps add modernizer.
https://css-tricks.com/using-webp-images/#article-header-id-4