In practice, such scripts are usually invoked from html files generated by other CGI programs, i.e., smi2gif references are machine-generated. In this demonstration file, they are hard coded to provide a simple self-demonstration.
Icons are are normal gif images, but smaller.
They are used here in a table, but could appear anywhere a gif image is allowed. A chemical table. The following table of 150x150 images shows the appearance of various colormodes.