1-Why can't we take images by using webcam?
I'm going to give you a spoon and a dixie cup. I want you to empty the swimming pool. In a torrential downpour. Let me know when you're done.
2-If we can't use webcam can we use a shield to do this work. I know there is one shield taking images.
Which shield? For what? You simply can't pipe all the data through the Arduino.
3-Or is it better to use some other thing to take images and send to web?
That's what a web cam does. There is no reason to try to pipe all the data through the Arduino.
4-Can we collect all of these data from arduino and send to web to access from other places?
You don't appear to understand web (client/server) data handling. There is no place called "the web" where you send data. You create a web server (the Arduino with Ethernet shield can do that) that serves up the data on request.
The clients (web browsers are the most common but not the only ones) request data as they need it. There are meta tags that the server can put in the served page that tell the client to fetch the data again after a suitable period. The client is free to ignore them or to respect them and pull the data again later, making what appears to be a real-time system (but really isn't).