Docs: trimmed and compressed the background image from >1MB to 250KB. Still far too large, but better.

- Reduced JPEG quality from 85% to 30% which is still thoroughly high enough quality. This reduces it to about 500KB.
- Trimmed background image from almost 4000px wide to only 2000px (this will affect people with horizontal window resolution greater than 2000, but should the rest even have to download such a large image?)