katswhiskers,
Hi,
To be frank, imagination will not help you a whit. Mindfulness and intention may. As for your Root Chakra being 'unbalanced'? What qualifications does your Teacher have and what, if anything, did he\she do that allowed such an assertion? Besides, the grounding technique you were told to do may not do anything for balancing your Root Chakra. There are other techniques for that sort of work.
I suggest that you keep working with the grounding technique you mentioned (it is a fairly common and standard exercise). Be mindful of what you are doing and be focused when you make the intention to extend yourself into the Earth for grounding. After some weeks of these practices see what you feel when you do Reiki with and without the grounding.
Cheers,
RC
