image updates for chapter 0.2

@jachiang please address the following:

- https://github.com/bitcoinops/taproot-workshop/pull/13#discussion_r320789551
- https://github.com/bitcoinops/taproot-workshop/pull/13#discussion_r324253922
- https://github.com/bitcoinops/taproot-workshop/pull/13#discussion_r324481755