Octocat image alignment, alt tags, and height values

Slide 4:

* `height="300"` instead of `300px` because the values are unitless
* More descriptive `alt` attributes, plus `title` attributes
* Octocats are aligned at the whiskers
* Presentation notes list the mascot components in logical order (cat + octopus = catopus, but octopus + cat = octocat)