Measuring electrical amounts on existing digital photos

Unless the photo was originally taken with special equipment - it is not feasible to determine temperature from a photograph. 

Heat information in photos (such as, false-colour images showing the places where most heat is leaving a building) are taken using purpose-designed equipment. This works by the camera 'seeing' non human visible infra-red radiation, and only that, coming from the subject.

Standard cameras work by seeing a different band of frequencies and include an Infra-Red Filter which blocks off this "heat" radiation from also being captured. The lens would focus that differently adding blur, also the intention of camera designers is usually to mimic the response of the human eye: and the human eye does not "see" temperature.

Some cameras have this IR filter specially removed, because the pictorial effect of capturing some infre-red is desired. In this case a different filter may be fitted which largely blocks visible light while still letting IR light through. This contrived system is still less exact at evaluatiing subject temperatures, than a purpose-made thermal camera would be. A night-vision device, too, often does reflect differences of subject temperature but again not in a calibrated way. There would still need to be some interpretation - no reading off a number directly.