image processing - Calculate mean and standard deviation from ImageHistogram

I am doing my PhD in Clinical Neuropsychology; I had someone helping me analyse images (.jpg pictures) in Mathematica, but it turns out this person made some really bad decisions, and well, now I have to do it myself. But, I have no idea.

Basically, what I want is to analyze the JPEG image, get a RGB histogram, and get the values for the mean and standard deviation of this histogram.

Using ImageHistogram [[*image*], Appearance, -> "Separated"] works fine, but I can't figure out the next step, turning the histogram plot into numbers.

How can I do that?

Thank you for helping!

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