Use 100% for image max-width

This commit changes `max-width: 640px` to `max-width: 100%` for images in
the .primary-content container. This will give the images some additional
width when the viewport is wider than 940px. Also added a comment about
a misguided ally related style block.
Merge pull request #228 from steveax/tweak-to-image-max-width

Use 100% for image max-width in .primary-content container