Thumbnails are usually stored in .THUMBDATA files.

THUMBDATA file format is used for indexing and caching processed images.

THUMBDATA files are generated so image previews (thumbnails) are quickly loaded and users do not experience any sluggish behavior when they browse through their images.

SD Card Thumbnail Finder will help you recover thumbnails stored in THUMBDATA files.

1.) You can scan your SD Card for thumbnails.
2.) You can pick a THUMBDATA file for processing

After Recovery you can share or save thumbnail to a choice folder.