I love the image resolution and volume of data that modern digital cameras offer. However, if you publish those digital images on a Web page, image size becomes a problem: If people can't download your Web page quickly, they probably won't download it at all. But with your favorite image-processing program and a few minutes of work, you can turn a 400KB monster into a dainty little 25KB download—with little decrease in image quality. You'll see the download difference even if you're viewing a Web page locally over an Ethernet connection.
First, you need to crop the picture. When I started my photography hobby in the early 1970s, I learned that a lot of good photography happens in the darkroom rather than while you're snapping the picture. With digital photography, your image-processing software replaces the darkroom, but that same principle applies. Does that outdoor shot of your family need the top 25 percent that shows just the sky? What do you want the picture to say, and what parts of the picture don't help tell the story? Use your imaging software's crop tool to trim the parts of the picture that don't add to the story. Be ruthless: Fewer pixels mean a better-focused story.
Second, you need to reduce the picture size. Most modern cameras produce JPEG images that are 1200 x 1600 pixels or larger. The biggest image that the Web can comfortably accommodate is 640 x 480 pixels. Reducing the horizontal and vertical dimensions by one-half produces an image about one-fourth the size of the original. Try to resize images by factors that divide the height and width into whole numbers, rather than fractions. For example, if you reduce the dimensions of a 1024 x 768-pixel image by one-half, the resized image has whole-number values—512 x 384. But if you reduce the length and width to one-third their original size, you end up with fractional sizes—341.333 by 256—which makes the resizing algorithm's job harder and results in less-sharp images. Also, examine your imaging program's resizing tools. For example, Jasc Software's Paint Shop Pro offers both resize and resample tools, but the resample tool creates a better-looking image.
Finally, you need to determine the optimal compression for your image. Experiment with JPEG compression to see how much you can compress an image before it starts to take on a blocky, stair-step look. At what point does that mosaic effect occur? Unfortunately, I can't answer that question because every camera behaves differently. When you take a picture, your camera receives that picture in a raw format that requires a lot of memory. Many cameras immediately convert the raw format to a JPEG format by using a certain JPEG compression level, and that compression level varies from camera to camera. When you set your camera to save images as fine or normal, you're setting the camera's JPEG compression value. But each brand of camera has a different definition of fine or normal, so I can't give you a rule of thumb for JPEG compression that works the same for all cameras. Experiment with different compression values, and you'll find the best one for your camera.
If you plan to post your photos on the Web, edit your images first so that viewers spend less time downloading them and more time remarking about your photographic expertise.