I've loaded an HDR image into my scene as an environment texture. However, I need to rotate the image, and so far, haven't found a way. I assume this is possible, but I need to know HOW.
In the World section of the Material nodes add a Texture Coordinate node and a mapping node to control the placement of the Environment texture. By changing the rotation on the mapping node you can place the HDR where you need it.
As of Blender 2.8x, the World section is part of the new Shader Editor:
You need to load the HDR image from the "environment texture node". The environment picture in your scene will not work alone.