I changed the links for both files. I found a website that will convert the link from a google photos image to a link that can be embedded:
https://ctrlq.org/google/photos/
The neat thing is that I can upload a large file to google photos, and then put in the width to display at the end of the link (w=1000) and the google server will provide the re-sized image.
Regarding the file extension, I'm under the impression that they are conventionally used, but not really necessary. The HTML code is <img src="https:.....> So the browser knows it's an image. My understanding is the server itself sends a MIME type (jpg, png, or whatever) in a header separate of the file, and so the browser knows what kind of image it is, and therefore the browser doesn't care how the file is named.