Optimizes image file sizes

I've noticed that there are some really heavy image files on the website. Especially the header images for each OTS chapter had some huge potential for optimization.

**Technique** (which is relatively popular) used to achieve smaller file sizes while keeping the perceived image quality relatively high:
- increase the dimensions of each image
- save as highly compressed jpeg file
- use ImageOptim to squeeze out some extra KBs

**The Effect:**
By doing this i was able to reduce the weight of the Berlin page from 789KB to 416KB. Which is about 48% less than in the current state of the site. The Dortmund page from 1.1MB (!) to 237KB, a reduction of 78%.

**Suggestion:**
An approach like this maybe should be promoted in the handbooks/guidelines to keep the steadliy growing OTS website as slim and lightweight as possible.