Last time I worked on a client project and he was fetching the strange issue with his image URL. Image URL was appending with https://i1.wp.com. Actually he was using the JetPack’s CDN for image optimization. So image was directly from JetPack CDN sever.
But problem was that some images are not loading on his site. So I went to JetPack settings page and disabled the following checkbox. See the attached image at below.
If you are really wanting to optimize your images, I shall highly recommend the ShortPixel Image Optmizer plugin. I am using it on my all sites. I am quite happy with it.