You can do this easily with a free program called GIMP. First make sure the program you are using, KompoZer, is actually resizing the images and not just putting code in the HTML file to resize the image there.
Start GIMP and load all the images you will need thumbnails for. Then, open a new file and set the image size to the size you want your thumbnails to be. You will have to do the following for each file:
1. Select the original sized artwork and press ctrl-c to copy it.
2. Go to the window for the thumbnail sized image and press ctrl-v to past the artwork into this file.
3. Using the tool that looks like a plus sign with arrows pointing in every direction, move the artwork around until you get the cropping you want.
4. Press shift+ctrl+s to "save as". Make sure the file type is jpg and pick a name that is the same as the large artwork, but with "-tn" or something added to it. It is very important to have consistency in file naming for when you put them in your web page.
5. A window that says "Export File" will pop up. Select "Export."
6. A JPEG quality window will pop up. Select whatever you want.
7. Repeat from step 1 with the next piece of artwork. Don't worry about pasting the new artwork on top of the last one. The image will be flatted and will only contain what you see.