In case you don't want to install an extension, search in about:config for "image.webp.enabled" and set it to false. Worth mentioning, there's also a "image.avif.enabled" entry.
EDIT: seems that this extension need to be updated, sorry...
But I don't know, if you allow javascript maybe you will leak your real user agent?
I like to switch between iceweasel and seamonkey, and I'm "proud" of my different and unique user agent :)
Speaking of privacy, actually I'm more concerned about referer, third-party sites, content delivery networks and so on.
