Convert large database of mixed-format files into screen size jpegs

If you have the technical skills to install Python3 and the Python Imaging Library on your computer, you can use this Python script:

# !/usr/bin/python3
import os
import sys
from PIL import Image

size = 128, 128

def thumbnail(fromFile,  toFile):
    _, ext = os.path.splitext(fromFile)
    if ext.lower() in ['.jpg', '.jpeg', '.png']:
      print('Creating thumbnail', toFile)
      im = Image.open(fromFile)
      im.thumbnail(size)
      im.save(file + ".thumbnail", "JPEG")

[_, fromDir, toDir] = sys.argv
for root, dirs, files in os.walk(fromDir, topdown=True):
   for name in dirs:
      toPath = os.path.join(toDir, root, name)
      print('Making', toPath)
      os.makedirs(toPath, 0o777, True)

   for name in files:
      fromFile = os.path.join(root, name)
      toFile = os.path.join(toDir, root, name)
      thumbnail(fromFile,  toFile)

Run it like this: python3 scriptname sourcedir destdir

Remember that sourcedir must be a relative pathname. If you have a file named /usr/traveler/mystuff/images/2009/Mar/IMG3.jpg, go to the directory /usr/traveler/mystuff, and use python3 scriptname images thumbnails, the output will be in /usr/traveler/mystuff/thumbnails/images/2009/Mar/IMG3.jpg

I can't stop recommending IrfanView if you use windows. Just be careful with the options you choose, specially Overwriting and deleting files, make some tests with dummy files and folders before running any batch script. Make a backup of your original files.

File > Batch Conversion / Rename

And in Advanced options, you can choose to create subfolders.

You can simply drag folders from the top preview window into the bottom list.

Irfan View recognizes a lot of file formats, and you can choose to export them all into jpg, define the quality of the files, apply some sharpening, etc.