Add picture to landing page

The code block: 

```
import climatecritters as cc
from climatecritters.model_critters import Lorenz63

model = Lorenz63()
output = model.integrate(t_span=(0, 50), y0=[1, 1, 1], method='RK45')
ts = output.to_pyleo(var_names='x')
ts.plot()
```

should be followed by a picture. 