Webp and webpage complete are different. Chances are the filename will tell you. If it ends in webp vs one of html, mhtml, htm.
In the case of it actually being a webp, as I mentioned before, just make the src="image01.webp" or whatever the actual name is, make sure you are including the webp extension. If you are fiddling with this locally, the fully qualified path may be needed. I believe most browsers support it at this point.
If it is one of the html types, jamming it into an img tag will not work, you need to find the actual image and save that.