Other people's work
Shame to say, there are so many cool functionalities I have never coded nor used in my websites, such as programmatically generate screenshots, generate animated gif using a sequence of screenshot, create captcha, in-progress animated image.
Fortunately, there are many pieces of working code up for taking on the web.
ASP .NET Captcha server control :
Very popular (meaning it has been widely tested)
Screenshot of Webpage with ASP.NET
Make use of IECapt.exe to capture screenshot of a web page given by an URL
Create an animated GIF using .Net (C#)
This seems to be a hard one. Most animated GIFs are created by using offline imaging software, however, I sometimes I really want to streamline this process, especially when I want to demo a running web page.
This one seems to work, though it has only given only minimal explanations.
Animated "Wait" image and long-running progress
There are so many progress-bar type of applications, however, this one is breathtakingly simple and ingenious.