VeryPDFPro v5.0 has a great feature which can be used to batch resize your JPG and other image formats on the fly. You can download and install Pro v5.0 from VeryPDF website first,
after you installed it, you can launch a cmd window, run following command line to batch resize your JPG files in "D:\temp\pic\" folder,
for %F in (D:\temp\pic\*.jpg) do "C:\Program Files\docPrint Pro v5.0\imgconv.exe" -sample 100x100 "%F" "%~dpnF-thumbnail.jpg"
Above command line will preserve the image aspect ratio, so it may not produce images that are exactly 100x100. To force this to happen, modify the original command to by placing an exclamation point at the end of the desired resolution:
for %F in (D:\temp\pic\*.jpg) do "C:\Program Files\docPrint Pro v5.0\imgconv.exe" -sample 100x100! "%F" "%~dpnF-thumbnail.jpg"
OK, you will get lots of new JPG files which size are 100x100 pixel.
imgconv.exe is a free tool in the docPrint Pro v5.0, you can use it free, you needn't pay for it.