According to PageSpeed Insights diagnostics, stated in "Reduce the impact of third-party code", listed under "googleusercontent.com". It shows a my blog header image together the post images. Why my blog images are considered as third-party code, and how do I fix this issue.
Where are you hosting the images? Are they on your site or on another site or subdomain? Side note: make sure they're webp format.
You're worried about pagespeed insights, and using Blogger... *SIGH* That's like trying to mate a big block crate motor to a 1984 Yugo GV. That said, if you could provide a link we could probably better diagnose your issues, but honestly it's Blogger. Their code is such a steaming dung-heap of incompetence worrying about pagespeed or lighthouse is utterly pointless. It's a cute toy for making blogs for grandma, not a serious business tool.