But I think the important thing that Allan needs to know is how to create a picture so that it is as small as possible.

This comes from two factors, DPI and file format. Problem is the following formula...

higher DPI = larger file size = clearer picture

You want your scan to show the most detail possible so you scan it at a higher resolution and therefore scan it at a higher DPI (200+).

The file format also determines the file size (based on the compression technique within the format). JPG is the best compression and some native formats are the worst.

Try scanning a photo at 100DPI (a decent resolution for the web) and save it as a jpg, then the same at 200DPI and save it as a GIF. look at the file sizes and the file clarity and you'll see what I mean.