OK, after posting the images above a few observations can be made.  The downsizing algorithm used by the website when doing the thumbnails is doing a fairly good job regardless of the original image size, but the only way to get perfectly sharp images is to upload them at a size smaller than 800x600 to make sure that the thumbnail is the original image.  So if you are happy with an image with the size of 800x600 or smaller then that's the best size to use because the thumbnail inserted into a po