recreate exif thumbnail

Exif

Exif is a standard set of headers contained within Jpegs (a thumbnail image being one of the headers, although there's lots of other info as well)
Most digital cameras write the Exif headers to the file, but it sounds like you don't have the headers on your files

Nautilus, I believe, can use the Exif information from Jpeg's to provide image thumbnails.

Try to find a program that can scan the images, and generate the Exif thumbnails

Never used it, but a google brought this up
http://www.sentex.net/~mwandel/jhead/

Looking over the online manual,
option -mkexif creates an Exif header for a Jpeg
and
option -rgt creates (regenerates) image thumbnail information and writes it to the Jpeg header