Fix broken images

[adamvoss]

There are a number of images in the guide that point to external sources or are otherwise broken. These need to be identified and should be fixed, removed, or replaced as appropriate.

NB. care needs to be taken with displaying external images to properly attribute them and indicate they do not fall under this repository's license.

[adamvoss]

@rocketjosh Regarding image sizing:

The original Markdown specification does not include image dimension specification. There are some efforts for an extension to support this, but none of these are [yet?] supported in GitBook.

The main options would be:

1. Add image dimension support to our Markdown files via a preprocessor step
2. Use css rules on the img tag to restrict the size to something sane (but would not give us full control)
3. Use GitBook templates
4. Use the HTML img tag.
5. Presize the images.

1-3 will not work outside of the context of the full GitBook rendering (i.e. generic markdown editors will be unaware). 3 is arguably the "right" choice within GitBook; but I am hesitant to commit to a GitBook specific feature until we are sure we are happy with it.
5 causes a quality loss which is not desired when you consider all the different devices and form factors that may view this.

That leaves using the HTML img tag directly to use the height/width attributes. This will be easy to [automatically] rewrite later if we choose a different solution, works will all editors, and will be portable to other Md-based systems if needed.

[adamvoss]

@rocketjosh has this been resolved?