The problem:
If you look at the docs for imread, the function takes an integer flag called imreadmodes. This flag seems to accept information about resizing the image, rather than changing color spaces.
The solution:
I believe you are looking for the cv2.cvtColor function which uses a flag to determine the source and destination colorspaces.
Both flags are simple integer enumerations. I assume the imread function is simply doing the best it can with the wrong type of flag.
You probably want to do something like:
BGRImage = cv2.imread(imageName)
YCrCbImage = cv2.cvtColor(BGRImage, cv2.COLOR_BGR2YCR_CB)