Relax.
Take some deep breaths.
I did not delete any thumbnails.
I found out something interesting.
I looked at what was in the thumbnail dir.
Then I went to another drive that had some family pics.
UM made thumbnails of some of those pics.
I am not complaining, I just like to learn. 
Yep, that’s how thumbnails work. There’s a piece of software running in the background (it’s called a daemon) that monitors what you access through Caja: when you open a directory full of pictures (or videos), it checks if the thumbnails exist in the cache for these specific files. If they do, it loads them and display them in place of the icons; if not, it creates them in the cache and display them.
In Caja prefs, you can adjust settings to create thumbnails for local dirs only or network too, to choose a filesize limit (files bigger than that will not have a thumbnail created − might only apply to network dirs, though) and so on.
The software that creates the thumbnails specifically is called a thumbnailer and you can install new ones for file types that are not handled by default, to create thumbnails e.g. for LibreOffice documents, for GIMP native (.xcf) format and so on.
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Thanks for the detail explanation.
I use Thunar instead of Caja.
Generally this isn't a good response to people who are taking time out of their day, and energy, to patiently try and help you.
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My comment was in response to your comment.
Actually the .cache folder is a normal part of how our OS operates. It should have nothing to do with a crash. I have cleared mine out in the past (obviously not lately), but there's really no point in doing so, unless your running out of space.
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I understand steven.
I am not clearing out the .cache folder since I learned how to set thumbnails to never in Caja.
The thumbnail folder is noticeable smaller.