The thumbnails showed up and no image, only the spinning juicebox icon.
Unfortunately, I cannot explain what might have happened.
You should certainly be able to edit a gallery without losing any gallery images. I've not encountered this problem before (and I don't recall any other users reporting such a problem).
I uploaded the gallery to my website, same thing.
It certainly sounds like the image entries were removed from the gallery's 'config.xml' file. If you upload a modified gallery folder to your website, then existing images will not be deleted (but if the image entries are no longer in the 'config.xml' file, then the images will no longer be displayed in the gallery).
If the problem happens again, check your gallery's 'config.xml' file (you can open it in a plain text editor) to see if the <image> tags for all your images are present.
Also, before saving your gallery on the 'Publish' tab, double-check that all the images are present on the 'Images' tab.
Now I can't delete the images directory (which I now can't access, probably why the browser didn't open any images, only thumbnails) from the first go-round. Needs Administrator privileges. Also some of the other directories need level by level deletion.
This is also very strange (although, as you say, if the 'images' folder permissions are too restrictive, then the browser will not have access to the image files).
Re-uploading images to your website should not change their permissions.
I do not know how you are uploading your gallery files but, in case it helps, I use Filezilla (a free dedicated FTP program) which I've used for many years without issue.
If you find that re-uploading a file (or folder) to your web server changes its permissions, then you might like to try a different FTP program to see if this makes a difference. An alternative FTP program for Windows is WinSCP (also free).
I'm not sure if these notes will help but if you can replicate the problem consistently, please post back with a set of steps that I can follow to try to reproduce the problem myself and hopefully help further. Thank you.