Provide an alternative to lena.png for two examples that use it.

I realize that lena is *the* standard image in the image processing world. However, I would think that most matplotlib users (myself included) are not aware of and/or don't care about standard images; and  that most people would object to the pornographic origins and nature of the image. The changes will allow these people to browse the gallery and not see lena. The original lena images remain in the example sample data directory for those who are interested.