Bug report

figimage does not plot image expected when the output file is pdf.

Code for reproduction

import matplotlib.pylab as plt
import numpy as np

fig = plt.figure()
img = np.random.rand(50,50)
fig.figimage(img, 20, 20)
plt.savefig('test.png')
plt.savefig('test.pdf')

Matplotlib version

• Operating System: Ubuntu 16.04
• Matplotlib Version: 2.0.2
• Python Version: 3.6.1