First download the thumbnail generator: http://www.shockace.co.nz/tools/

Change the downloaded file name (PhotoResize200N.exe) to suitable thumbnail size.

Example: If you want to make thumbnails with 300px maximum width and 300px maximum height from a larger images, change the file name to PhotoResize300N.exe (Note that file name is case-sensitive)

Do not run the downloaded file, just select and drag all your image files on the downloaded file as if you are moving the files to a folder. This will generate thumbnail images on the same folder as the original files.