What is "energy" in image processing?

If you consider that (for natural images captured by cameras) the light is an energy, you may call energy the value of the pixel on some channel.

However, I think that by energy the books are referring to the spectral density. From wikipedia:

The energy spectral density describes how the energy (or variance) of a signal or a time series is distributed with frequency

http://en.wikipedia.org/wiki/Spectral_density

Energy is like the "information present on the image". Compression of images cause energy-loss. I guess its something like that.