Thie video tutorial below is an introduction to optimizing images for the web using Adobe Fireworks. It can take some practice to get a sense of what is a good level of optimization for an image. Each image is a little different, and depending upon the role that it plays on your site, you may want to make some concessions in file size where you need an image to be very high quality. The opposite case may be true as well, in certain circumstances.
The more practice you have optimizing images, the better you will get. You will also develop a stronger sense of which file type (JPEG, GIF, or PNG) is right for particular images.
There are many other tools for optimizing images that are better than Fireworks – for bulk optimization particularly. Smushit and PNG Crush are just a couple of tools to explore for those who want more advanced optimization tools.