If you don`t want to use the automatic thumbnails, here`s what you need to do:

- xml/gallery/config.xml - set useOwnThumbs to true
- xml/galleryMenu/config.xml - set useOwnThumbs to true
- Put a folder called thumbs inside each of your gallery folders. The thumbnails must have the same file names as their large images

* note

You will still need to set the thumbWidth and thumbHeight. You might also have to use the thumb thumbXScale and thumbYScale in the gallery menu config file. This is because your gallery thumbs and menu thumbs are likely to be different sizes. For best results set useOwnThumbs to false for the gallery menu. 

please leave feedback on the file page. 

enjoy,
Chuck