Replace Lena with Hopper (Part I)

For #884.

Partial replacement of Lena test image with one of Grace Hopper. I'm sending this Partial PR so as to enable new tests to be written based on the new image in the meantime. Another PR will complete the replacement.

 * hopper.jpg added, cropped and resized from http://www.history.navy.mil/photos/images/h96000/h96919k.jpg

 * Some other hopper.* images added based on this, and some lena.* removed. 

 * Some tests updated to use hopper.

 * `hopper()` added to helper.py, alongside `lena()`.