The whole visible webpage (without scrolling, just whatever is visible at a point), I draw an area on that using my mouse (either by drawing or by just selecting a two points as a rectangle).
I click a submit button, and that visible area is converted as an image and saved to the server / client computer or whatever way, but I get the image.
I just want to know whether this is technically feasible, if yes, how ?
Yes, maybe you could try using jsFeedback? I know Google does this as well.
For security reasons, No. If you're on an intranet or want to use it only privatly you may research into making a custom browser plugin that can do this for you.