So is my imagination a sin?
Your imagination has nothing to do with the hebrew intent of the word translated to imagination.
"yetser" or Form. The form of our heart is evil sounds really harsh, the reason the old testament seems so harsh is that the language is very descriptive in order to capture attention.
Well it works, but we have to filter all of that through GODS LOVE. So what does a harsh sounding term mean when filtered through Gods love.
It simply means that we do not measure up to Gods greatness RIGHT NOW.
Your imagination is totally separate from that. You use your imagination to write stories to reflect a greatness of God right? Well, if we can use our imagination to reflect deeply on God then how can it be a sin?
So the final answer to the million dollar question is NO and God is your lifeline so you've WON.