The image below is the 9-year WMAP image (2012) of the cosmic microwave background radiation across the universe (from Wiki). As I understand it, the major axis represents the equator and the minor axis the central meridian. But, what is the orientation of the equator and central meridian relative to the earth ? In other words, how do I know where I'm looking ?
The usual convention is to use the equatorial coordinate system, so the major axis of the ellipse represents the projection of the Earth's equator. The minor axis of the ellipse is usually right ascension = 0h or 0°, but I guess RA = 12h or 180° is also a reasonable choice.
Oops! As Thriveth mentions in the comments, it appears that the usual coordinate system used for CMB is the galactic coordinate system, which is a spherical projection centred on the Sun. The equatorial plane of this projection is approximately the galactic plane of the Milky Way, with the centre of our galaxy (the radio source Sagittarius A*) in the centre of the map.
It's difficult to determine the true galactic plane, since that's dependent on the distribution of all the mass of the galaxy, so the galactic coordinate system is defined in reference to radio observations of galactic neutral hydrogen through the hydrogen line.