ImageMeasurements

ImageMeasurements[image, "prop"] returns the value of property "prop" for the entire image.

ImageMeasurements[image, "prop", format] returns values in the specified output format.

Examples

Get mean intensity:

img = ExampleData[{"TestImage", "Lena"}];
ImageMeasurements[img, "Mean"]
(* 0.435 *)

Multiple properties:

ImageMeasurements[img, {"Mean", "StandardDeviation", "Max"}]

Please visit the official Wolfram Language Reference for more details.