Reducing the size of the tutorial images

Reduced the size of the tutorial images from 2.6 mb to <700k. I did two
things — resize the finish-cat and introduction-cat images to the size
that is forced by the CSS && run pngcrush on all of the images.
Changing two images to jpg

finish-cat and introduction-cat are both better off jpg.
Swapping the images, and updating any references to the images in the
.txt files.
Merge pull request #183 from dh--/master

Reducing the size of the tutorial images