To see the mean intensity of an entire image, don't select anything (or press Ctrl+Shift+A to select None), then Ctrl+M. Or go to the Analyze menu and select measure. You'll get the Area, Mean, Min, and Max values of the entire image.

One important notice: beware of "burn" regions on image. I such regions, for 8-bit image for example, you will get the value 255, although real value is maybe 300 or so.

You can also write simple Matlab code, if you have lot of images you are doing routinely, as the intensity is nothing more than simple sum of elements in matrix (inage).

Thanks you Sir....