What does 'energy' in image processing mean?
I quote from here which our friend has provided.


  Energy in this context is not, strictly speaking, the same as the conventional notion of energy in physics and the other sciences. The two concepts are, however, closely related, and it is possible to convert from one to the other:





  where Z represents the magnitude, in appropriate units of measure, of the load driven by the signal.


Also for figuring out the interpretation of energy in this context you can see here.


After a while, I found a bit better explanation for energy as you can read the answers from here. 


  Energy is used to describe a measure of "information" when formulating an operation under a probability framework such as MAP (maximum a priori) estimation in conjunction with Markov Random Fields. Sometimes the energy can be a negative measure to be minimised and sometimes it is a positive measure to be maximized.


or you can read:


  Energy is a fairly loose term used to describe any user defined function (in the image domain).