If in the first example under http://matplotlib.sourceforge.net/mpl_toolkits/axes_grid/users/overview.html#imagegrid the "aspect" argument of imshow() is not equal to 1.0, the images are irreversibly squashed in one direction when the window is resized.

To reproduce, for instance change
grid[i].imshow(im)
into:
grid[i].imshow(im, aspect=1.3)