I am trying to build an web image server. It serves images to lots of clients(10 thousands+) simultaneously. (It will be a easier problem if there is fewer clients.) What is a good way to do so, with time delay as small as possible.
I am new to this field. Any suggestion will be welcomed.
How are the images to be served? Are the images generated on the fly? or are they static and stored as .jpg or other format on the file system?
Either way, I'd use ASP.NET .ashx (generic handlers) and use the System.Drawing classes.
You'll also want to setup TCP/IP Network Load Balancing per http://support.microsoft.com/kb/323431
With that many clients, you may want to look into using a content delivery network (such as Akamai). On the surface it might seem expensive, but if you really look at the cost of maintaining the hardware and particularly the cost of bandwidth, it starts to make economic sense.
Definitely look around for a good delivery service. Akamai is the best known.
if you really want to do it on your own, forget about Apache/IIS. much more appropriate are 'light' webservers. Two very good are lighthttp and NginX (wiki). NginX in particular, has a really solid performance.