I simply use the IrfanView for my thumbnails which is free to download. I just open the image as jpg and select the image area where I want to crop by dragging a square equal box with the mouse and crop the image, after that go to resize the image and change the pixels to 200x200 and save it in the same directory where the original image exists. I name the thumbnails  with T-  such as T-imagename  so that I know T-  is for thumbnails.
I always post work my images in TIF format in PhotoShop when it's done I save a copy of it as jpg to post it in my gallery but PhotoShop saves images with too many kilobytes so the image becomes large like 800-900 KB even sometimes 1 MB this takes longer time to open them in the gallery so what I do is open the jpg image in IrfanView and save it again (over write) as jpg file, this will reduce the size of the image and becomes around 200-300KB  viewing it in the gallery will be faster. I don't worry overwriting the jpg image since I always keep the original rendered image as a TIF file.