thumbnails are smaller versions of images nautilus uses to quickly display a preview when opening a folder with images (or videos) in it.
They should be in your home directory ~/.cache/thumbnails,
and in my experience, can be safely deleted.
So one option would be to simply remove that directory, nautilus will then generate them again when needed.
To do so, you can open a terminal and type
rm -Rf ~/.cache/thumbnails.
Of course, this will lead to some loading times until the cache is refilled.
This solution would not require root privileges.
Virusses are rare on linux, but being wary of giving root privileges is a good thing. I would not think that you have a virus, but the only way to know would be to scan. I have used clamav for that purpose in the past.
Why exactly nautilus is having this problem I cannot tell, wiser souls will have to answer this one.