You can change the skymap under the "Sky" tab of the Render Controls dialog. There are several different types of sky map to choose from. Chunky supports a few different kinds of skymap projections, however "equirectangular" projection is the most common type of sky map.
It is recommended to use a large skymap in order to avoid the sky looking blurry. However, if the skymap is too large it will take a long time to load.
Equirectangular skymaps can have 90° or 180° vertical resolution. With 90° vertical resolution the skymap will only cover the sky above the horizon, so it is mirrored at the horizon. If the skymap has 180° vertical resolution the mirroring is not needed: the skymap will cover both sky and ground.
Where to find skymaps
These pages provide links to other pages, or downloads of non-HDR skymaps:
HDR skymaps have the advantage that they can be used to provide realistic outdoor lighting for a scene.
Most of these sites provide paid-for skymaps, but many of them have a few free samples available:
You can also try searcing Google for Panoramic Sky Texture
Render a Sky Map with Chunky
Chunky can be used to render a sky map. You will need to follow these steps to set up the camera correctly to render a full 180-by-360 degree sky map:
This shows the Camera tab with correct values entered:
And this is what the resulting render should look like: