Below you will see various attempts at creating a pixelization effect like you would might see on television.
This first example allows you to adjust the pixelization with the slider. If you zoom into the image, the pixelization will remain constant - it adjusts with the zoom level.
This example is like the previous example; however, it does not make adjustments for zoom level.
This next two examples allow you to move a circle over the image. The pixelization has been clipped by the circle.