At this point, it looks to me to be a clear majority want the larger thumbnails.
I'm going to revert to them immediately, but leave the poll open. As a compromise, I'll allow 8 images and see what happens. I've not been able to find a way to remove the image links from a reply to a post.
I also have the ability to turn off the thumbnail and show images full size.
While I was fooling with the image controls, I noticed a option that is checked to show the thumbnails as PNG format images. I did a little research, and the Simple Machines Manual says that's a higher quality setting. It just might be responsible for the frequent reports of poor colors and artifacts noted in uploaded images, so I will experiment with turning it off to see what we get. It is also possible that showing the image full size might improve the quality, the resize option is turned on now. I definitely don't want to bring down the forum by playing with the options too much, but it might be interesting to try settings for a few minutes to see what happens.
I'll first try turning off the png. Generally, the changes only apply to new posts and are not retroactive.
Anyone who wants to compare send me a PM and you can try a before and after.