images hide axis lines

Hi,

I note that in the in the examples which display some images (e.g,
image_origin.py) the displayed images hide one of the x-axis (top one if
origin="upper" and bottom one otherwise). It seems like it may be
necessary to apply an offset before displaying the images (I looked
throughout the imshow.py code but didn't find the exact place to do it, it
may be a problem of offsetting and scaling).

Cheers,

  Eddie.