Good imaging means you can close your eyes and point to where each instrument is in the stereo field — not just left/right, but also depth (front/back). Imaging depends on: precise frequency response matching between left/right speakers, low phase distortion, minimal early reflections in the room, and proper speaker placement in an equilateral triangle.
Headphone imaging is fundamentally different from speaker imaging — with headphones, sounds appear inside your head (lateralization) rather than in front of you (externalization). Open-back headphones generally image better than closed-backs. Crossfeed plugins can improve headphone imaging by simulating speaker crosstalk.