What is a Stereo Image?
A stereo image is the perceived special location of various sounds within an audio signal.
For example, when you’re listening to a piece of music that paints a clear picture of instruments playing in a specific location throughout the track and their location to the listener. Songs with this quality are considered to have a good stereo image.
Just like in the 3-dimensional world, your stereo image has three distinct dimensions.
The height of your mix refers to how high your mix sounds. Height is achieved through proper leveling and EQ.
Width is how your sounds move from one side of the mix (or speakers) to the other. Width is achievable in many ways (more on this below), but one of the most important keys for width is panning.
The depth of your mix depends on how you use time-based effects like reverb and delay to create a deep or shallow image.
It's important that you consider all three dimensions to create the widest possible stereo image. You can't think about one dimension without considering the others as well - they all interact to create the total stereo image.