I think this is for the image located in the src="" and probably Google’s thinking the .png could be switched with .webp for the savings (not total savings from all of the images on the URL, but rather on this particular?).
Nevertheless, even if we are using a Pro plan and Polish & Mirage options, I remember Google still thrown the warnings/errors for the jpg images which were delivered and rendered as a webp in the visitors Web browser.
So, kind of “a gap” between the “insights results” and the working things for the website visitor.
You are also missing alternative text → SEO
I am not sure how Google determines this, by the screen width, or device pixels ratio, and compare to the image size if it fits?
Or rather Google expects it to be <picture> element with default <img> (due to the fallback) and then <source> properly defined with the srcset, media and type attribute so it can pick wheather it needs for “Desktop” vs “Mobile” testing …
I am just guessing …
Otherwise, maybe it saw some metadata from the image(s) could be striped out and that way saved few bytes as well?