Homepage - 2nd content update - 1st of August

- First and Second promo slots content update
- Remove unused old image

https://www.pivotaltracker.com/story/show/74435858
I'm not sure screen readers will use the heading below - they're not nested in the same element. I believe an `a` element containing an image with a blank `alt` attribute will read out the URL of the link instead of nearby text. (refs #559)

This might be resolvable by using `aria-labelledby` or similar.