Finding n largest coefficient to local energy ratio

I want to the n largest coefficient to local energy ratio. These coefficients are gained by applying PCA to an image and I have decided to take 5 largest coefficients. But what is the meaning of energy of an image? How to calculate it in Matlab using the equation shown in the attached image?

[John D'Errico]

Please don't post the same question multiple times, just to add some information to the first one.