Replace usage of Lena image in the gallery.

Note that lena.npy was created in PR matplotlib#1924 before I realized matplotlib could read png images without PIL. That file wasn't used anywhere else, so I remove it here.

Author: tonysyu

examples/images_contours_and_fields/image_demo.py

@@ -1,12 +1,11 @@
 """
 Simple demo of the imshow function.
 """
-import numpy as np
 import matplotlib.pyplot as plt
 import matplotlib.cbook as cbook
 
-image_file = cbook.get_sample_data('lena.npy')
-image = np.load(image_file)
+image_file = cbook.get_sample_data('ada.png')
+image = plt.imread(image_file)
 
 plt.imshow(image)
 plt.axis('off') # clear x- and y-axes

examples/images_contours_and_fields/image_demo_clip_path.py

@@ -1,18 +1,17 @@
 """
 Demo of image that's been clipped by a circular patch.
 """
-import numpy as np
 import matplotlib.pyplot as plt
 import matplotlib.patches as patches
 import matplotlib.cbook as cbook
 
 
-image_file = cbook.get_sample_data('lena.npy')
-image = np.load(image_file)
+image_file = cbook.get_sample_data('grace_hopper.png')
+image = plt.imread(image_file)
 
 fig, ax = plt.subplots()
 im = ax.imshow(image)
-patch = patches.Circle((130, 130), radius=100, transform=ax.transData)
+patch = patches.Circle((260, 200), radius=200, transform=ax.transData)
 im.set_clip_path(patch)
 
 plt.axis('off')