To load the network and to test on a single image, you need to execute the following lines of code. Here is the full source code:

import matplotlib.pyplot as plt 
import tensorflow as tf 
imort numpy as np 
import mnist_data 
 
mnist = mnist_data.read_data_sets('data', one_hot=True) 
sess = tf.InteractiveSession() 
new_saver = tf.train.import_meta_graph('saved_mnist_cnn.ckpt.meta') 
new_saver.restore(sess, 'saved_mnist_cnn.ckpt') 
tf.get_default_graph().as_graph_def() 
x = sess.graph.get_tensor_by_name("input:0") 
y_conv = sess.graph.get_tensor_by_name("output:0") 
image_b = mnist.test.images[100] 
result = sess.run(y_conv, feed_dict={x:image_b}) 
print(result) 
print(sess.run(tf.argmax(result, 1))) 
 
plt.imshow(image_b.reshape([28, 28]), cmap='Greys') 
plt.show()