usually, when we talk about 'imaging', we are referring to 'stereo imaging', i.e. the impression that certain sounds are positioned in certain places between he left and right loudspeakers (or in this case, ears!)

It's quite difficult to pin down objectively, but there is no doubt that some combinations of transducers produce a clarity of localization, with sounds easy to pinpoint accurately in space, whereas others produce a certain 'woolliness' or uncertainty of (apparent) positioning.

Many technical factors come into play, and the interactions between them are complex, subtle, and not necessarily terribly well understood.