Hypercyber   The boing demon   CGI programs?

CGI programs?

CGI stands for Common Gateway Interface CGI prorams were intended as a way to pass information back and forth between a http server and a different system like gopher or wais.

Like all other URLs, the URLs that provide the src of an embedded image can point to a CGI program that (for example) generates the image on the fly: 

The client can't tell the difference between this dynamic case and the static case where the server reads the data from a static file; to your browser, it's all just a stream of typed data.

This is how some people implement counters in their pages: the counter is a gif that displays a number; the program that produces the gif counts (e.g. in a file) how many times it has been invoked, and draws that number as an image.  (HTML has no provisions to include dynamically created text in a page; but images are always possible.)