Remember, we calculated that you want about 1500 by 1000 image dots in order to print your image. This works out to about 500 by 500dpi. However, scanning at 500dpi hopefully got more information off of the smaller portion of the original print, resulting in the best possible image quality for this case. Because a printer isn't able to accurately render a wide range of colors, you are forced to use high contrast images with relatively high resolution to get something useful. This works well on the Internet where big files are too slow, so image size becomes the more important consideration.
Resolution is traded off in favor of small image size.
The monitor you use to view your pages is able to do a good job of displaying a low resolution image, so it works out in the end. Personally, I usually scan at no more than 200dpi. When I know where I'm going to use the image, I enhance contrast and brightness. Wrap-up Knowing a little about the resolution and abilities of your output device makes it easy to calculate the scanning resolution you need. The tiny bit of effort involved gives quick and consistently good results.