Image ALTs not changeable?

I have run SEO Checkup Reports, in which I am shown that my website has several image tags which do not contain the "alt" attribute.

Now, I have gone through every single image stored on my server and added "alt" attributes.

The URLs in the report start with "https://i1.wp.com/insp***.biz/", which makes me think that these are URLs from the JetPack CDN (maybe), and not those on my server.

Any help in how to solve this is much appreciated. Thank you!