@Kevin -- good to know, but I don't know how to do tables and have 100 images.
@Turtle, thanks. I tried it, but it is left-aligned, both image and text. (And I validated the CSS, and it was good without errors.) I'm sure it's user-error... I just don't know what I'm doing. Regardless of how I tried tweaking the CSS, I can't get it to center. But this happened with the code I linked to above too, so again, it has to be me.
What I
do know how to use is Photoshop. And if I go that route I won't have to use separate code for mobi vs epub or worry in the future than some code I used becomes legacy, since I don't know enough about it to begin with and certainly do NOT want to revisit this book in the future and have to re-do 100 images. This has been a 4yr project I cannot wait to see in the rear-view.
It's a shame the figcaption method doesn't work in mobi as it looks beautiful in Sigil with nice crisp text. The Adobe method is not as crisp, but at least it will be in the right place regardless of device or passage of time.