I have a Wordpress page which is showing gallery that I created by using Wordpress built-in post editor: into it here is its url: http://jalrosh.com/lylescenter/?page_id=8
Now the issue is that when I click on some image its big image opens in a new page and that is disturbing page design. It should come in the central area but it is appearing on the top of the remaining page.
There are several plugins that could help you do this. For example http://wordpress.org/extend/plugins/lightbox-2-wordpress-plugin/ I haven't used this plugin, but a wordpress plugin search for "lightbox" came with several options. Try some of them out and see what one you like best.
I haven't tried any of them but supposedly they should take any image and have it viewed via a lightbox (which is the correct term for having the image displayed over the existing page).