The only thing I would add to the excellent advice you've already received is to make sure the final file size of the thumbnail is under 15K. Whatever program you use to resize should be capable of reducing the jpeg quality, which is equivalent to increasing the compression. Different programs list it different ways. I've found that it's better to reduce the file size by reducing the jpeg quality than to reduce the image size. I almost always make my thumbnails 200 X 200, then reduce the quality enough to get the file size down to 15K.