Update screen image - callback?

If this question has been asked and answered before, I'm sorry, but I looked and could not find it.

I want to modify the screen in an app. What I need is to get control in my app just before the screen is updated.

I will need access to the image that was going to be shown, so I can modify it, then send my modified image to the screen.

Any help at all will be appreciated.

Thank you in advance,

Peter Loeb