How to properly mean-center my data
Each channel of rgb colors has values between 0 and 255.
There are many normalization methods. For this case I would just divide the images over 255 without subtracting the mean so that I make sure the output is between 0 and 1

Z = X/255

if you are interested in centering the images around zero, you can subtract 127 (as a mean) and then divide over 255

Z = (X-127)/255