We are always imagining and meditating on something, whether we realise it or not. Most of the time we are imagining what the world tell us. Any and all imaginations that are not in line with the WORD of GOD is sin and will cause destruction (2 Corinthians 10:5).
Therefore, always be vigilant to imagine and meditate on good and noble things (Philippians 4:8) and the promises of GOD.
Imagining and meditating on healing is of CHRIST (1 Peter 2:24).