Depending on how "over" "over the existing page" is. You might wanna use a lightbox technique.
A very lightweight and crap-less lightbox (is: a module dialog that opens over the page, darkening (or even blurring) the exising page below it) … is shadowbox.
Read more at: http://www.shadowbox-js.com/
-> on download you can choose what you want.
ie. if you just wanna use it for an image, and already use jQuery then:
> custom build (adapter: jQuery; player: img; lang: english; untick Shizle.js)
And you’ve got a small pack just for that what you need.
If you need any further help implementing it, check usage-page and then ask here.