matplotlib: Stretch image to cover the whole figure

I did this using the following snippet.

#!/usr/bin/env python
import numpy as np
import matplotlib.cm as cm
import matplotlib.mlab as mlab
import matplotlib.pyplot as plt
from pylab import *

delta = 0.025
x = y = np.arange(-3.0, 3.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
Z = Z2-Z1  # difference of Gaussians
ax = Axes(plt.gcf(),[0,0,1,1],yticks=[],xticks=[],frame_on=False)
plt.gcf().delaxes(plt.gca())
plt.gcf().add_axes(ax)
im = plt.imshow(Z, cmap=cm.gray)

plt.show()

Note the grey border on the sides is related to the aspect rario of the Axes which is altered by setting aspect='equal', or aspect='auto' or your ratio.

Also as mentioned by Zhenya in the comments Similar StackOverflow Question mentions the parameters to savefig of bbox_inches='tight' and pad_inches=-1 or pad_inches=0