Fix image sizing for icons in chapter 4

[cansavvy]

https://leanpub.com/courses/jhu/documentation_and_usability/read/5#leanpub-auto-major-components-of-good-documentation

Icons should be small but they are not.

[cansavvy]

I think this is leanbuild thing that needs to be fixed.

[cansavvy]

I emailed Leanpub about this because I am stumped. Will report back on what they say. May just have to take those icons out in the Leanpub version.

[cansavvy]

Leanpub got back to us and image resizing isn't working for them either and is a problem on their end. They are going to work on it.