Question about thumbnail cleanup.

[MarkE]

I use ccleaner for Android every few days to clean up the files on an old Nexus 7 tablet, and it lists the stats and size of the files it marks for deletion when I analyze the devices.  I recently noticed that it lists "thumbnails" in the files that can be cleaned.  The amount it quotes is over 10 gb!  I don't have a lot of photos on my tablet, and the only apps I use regularly that might be saving thumbnails of images somewhere are Feedly and Reddit Gold and Platinum.  Is there some way I can view some of the files I would be deleting, to make sure that I'm not completely wiping most of my storage by selecting thumbnail files for deletion?  I'm considering doing a full backup to a flash drive in case something goes horribly wrong....