The problem is the popup composition window for launching a new thread. You can't resize it to make it larger. Google's web coding is such that that operating system functionality has not been enabled on that window. Any image you insert in line as Best fit is scaled to fit best in the small popup composition window. This gives you the woefully shrunk image you can see in the the first post above.
So here's what I will do from now on should I be posting any new thread with images in line.
1. Launch the new thread the usual way.
2. Give the thread a meaningful title but post just a single character in the first post, probably just a period.
3. Add a second post to the thread & create what I really wanted to put in the opening post of the thread.
4. Delete the first post with just the period.
The in line images will be a decent size. Unfortunately, the thread will forever after start with an ugly "Message deleted" notification. Oh well. Can't be helped. This would probably not even be an issue if it were still possible, like it was in the old days, to edit one's own posts. But Google removed that extremely useful feature & no amount of complaining since they did that has had any effect.