I recommend against imshow for this purpose.
Instead call image(). When you do, either pass in the two optional positions before the image data, or else use the XData and YData name/value pairs.
The positions to pass in should be the data coordinates of the centers of the lower left and upper right pixels. Those values would be similar to the min() and max() of your x-axis and y-axis values that you use for the quiver call.
