Nail_It is a program that can take the majority of images and make high quality thumbnail shrinks
Browse to the folder containing all the images you want to shrink, then browse to where you want to save the output thumbnails. Adjust the target width and height as desired then hit Execute! DONE!
-Make a compatible gui for single selection. I made this to do a huge amount of images at one time so this simple feature was never implemented. When I have the time I will add it in for sake of completeness