Adding a google pixel to a Contribution Page
You have two options:


Set up a sidebar in your WordPress theme that only appears on contribution pages that contains the pixel.  I know how to do this in Drupal using Blocks; I don't know how to do it in WordPress off the top of my head, but I'm sure it's doable.
If you are or have access to a coder, you can write a CiviCRM extension to do this.  I've written one for Facebook Conversion Pixels for successful donations.  My guess is you could modify this pretty easily, or hire someone to do so.