how to insert image inside an existing image hi i'm working on my sample site.in that I've done the background image blue sky into the respective region how can i add one more image in center of that like below ? and more over 
I've to add an web form in that center image i'm going to add? how can i achieve this?
It depends on your theme which you're using. Normally you've to create a block with image in its content and assign to the right region.

If you don't have the right region to assign to, try changing the existing content, or expand your theme and add new region based on your needs, or use Panels and replace your current page with custom layout (e.g. by using a flexible layout).