How are data in tensorflow.examples.tutorials.mnist formatted?
Based on the official documentation after loading the data,

from tensorflow.examples.tutorials.mnist import input_data
mnist = input_data.read_data_sets("MNIST_data/", one_hot=True)


You can use the following line,

mnist.train.images.shape


to see the shape of images, as you can see it is a rank-2 matrix which means images are unrolled which one dimension specifies the number of examples and another specifies the number of pixels __features.