A thumbnail is nothing more than a custom icon. To delete an individual icon, click on the file in the Finder and choose Get Info. Then click on the icon and hit Delete.
Or, because these are all images, we can cheat with a Terminal command. (Note that this will strip the file of custom icons (hooray!), Spotlight comments, and possibly EXIF data. If those are meaningful, seek another course of action.)
/Applications/Utilities/Terminal. Type in these two commands:
cd /replace
this_with_thepath/to/your/files
For example, if your files are in your Pictures folder, type:
cd /Users/your-short-name/Pictures
Then:
sips -rotate 360 *
Sit back, and all of the custom icons will be stripped from every JPEG and GIF image in the folder.