Optimizing isn't only to get your file sizes down. It's also to optmize for the information structure of the image. That said, reducing your file sizes is still a good idea in terms of bandwidth. While an individual connection may not use much to download your images, you would (presumably) like to have multiple people connecting to your site, and bandwidth savings can add up.
The JPEG format is optimized for photographs which generally have soft gradations between adacent pixels. The JPEG scheme creates a mathematical formulation of an image in blocks of 8x8 pixels where high frequency information is thrown away. The amount of information that is discarded depends on the compression/quality level you choose.
Line art, which generally consists of sharp color boundaries and lines is not at all well-served by the JPEG format. If you save line art to a .jpg file, you will lose the crispness of the art. Clean, solid colors will become muddied. For line art, an indexed color scheme such as the GIF or PNG format is much better. In these schemes, the color of each pixel is set. For the GIF format, efficiency comes from "run length encoding," where horizontal blocks of pixels can be set to the same color. On the other hand, the GIF format is limited to 256 colors, and so is rarely useful for photographs.
Use the proper format for the content type.
I'd like to add that optimization is still a concern for at least a couple other reasons:
- Not everyone has high speed Internet access, as much as we might like to think so. Many people still do not own their own computer, much less are able to afford the cost of a high speed connection.
- The "web" isn't just about loading web pages on a PC. There are a multitude of devices capable of surfing the web - and they do so at different connection speeds
- Even if a user is accessing a web site from a fast computer with a high speed connection, many web sites are more than just text and graphics; they have backend systems which control the data displayed. Anything you can do to speed up this process is a good thing.