When a picture is scanned and converted to digital format you have a choice of the resolution obtained up to the limit of the scanning device you are using. To speed the display of images, use the lowest resolution that still yields the required quality. The forth determinate of file size is the level of compression. GIF files are limited to 256 colors and are therefore used primarily for icons and other graphics with distinct colors. JPEG also allows you to set a compression level.
High compression produces smaller files but poorer quality images.
You will have to try several levels of compression with an image to determine the highest level that still produces the required quality image. Programs such as WebGraphics Optimizer assist in this process and can be downloaded over the web. When scanning an image you might try using 200 to 300 DPI which will yield a large displayed image relative to the original. A good original will yield surprisingly good display quality even at a high compression level. This allows the browser to continue downloading text while the image is being tranfered.