Problems with https

Converted from John's card:

Fix problems viewing over https at https://www.gafferhq.org/. It seems that images are missing - perhaps because they're linked via http? The console in Firefox has this to say, among other things :

```
Loading mixed (insecure) display content “http://image-engine.com/wp-content/uploads/2018/04/101_040_600_AO.1006.jpg” on a secure page[Learn More]
```