Add stock images and upload them via CarrierWave (refs #362)

All three images are licensed under CC and taken from:

* https://www.flickr.com/photos/sicarr/4277922732
* https://www.flickr.com/photos/mathias-erhart/2562106102
* https://www.flickr.com/photos/donnagrayson/1539610421

I added attribution to the images - so I hope we're on the safe side.