I hope these tips will help you. I cite advice I gave on several lists. If you know tricks you want to share, email me. Here is some advice I gave in several mailing lists.
People resize to reduce size which makes the images blurred.
Just go ahead and sharpen it. It's that easy. Key to make great thumbnails. Next, if you don't need animation or transparency, you'll get smaller file sizes with jpg than gif in most cases. Save either way and check file sizes.