title: Opening Image using imageio.imread in python
body: <p>In python, I use <code>style_image = imageio.imread('image.jpg')</code> to read an image, then I print the shape of the image <code>print(np.shape(style_image))</code>, I find that the size is <code>(216, 154, 4)</code>, why it is 4 instead of 3 because there should be 3 colors only.</p>