Location of thumbnail image

I have some jpg images which I edited and saved but inadvertantly saved over the originals.
Opening the image file gives the edited image.
The preview shows the edited image.
But the thumbnail still shows the original.

How can I retrieve the original from the thumbnail image? Where are the individual thumbnail image files located? I assume they are in Thumbs.db, but is there a way to get the individual images out of this file?

Thanks for any advice,
Janet
Windows 7 Home Premium 64-bit
Windows Explorer