What to do with the unbidden?

For the past week or so I've been haunted by an image that doesn't yet exist. It's been lurking along the edges of my thoughts during my workday and coming to the fore at that time just before sleep overtakes me. Today it got to the point where I couldn't wait to get home and try to capture this idea on canvas.

But the image refused to appear, and ultimately something else that I'd not planned or considered expressed itself.

It was a very strange experience, like my brain and hands were disagreeing, with the hands saying well, we don't feel like doing that so we're doing this instead. I've often done painting sessions where I've no idea where I want to go, but this is the first time I've known my destination and been thwarted.

So now I have the rough beginnings of something I didn't even know wanted to exist, and I feel obliged to shift focus and build upon these fine new bones. Who knows what it will eventually become?

At the same time I'm wondering if I've the discipline to set aside my original idea, and come back to it later. Part of me is scared that if I don't get it down, it will eventually fade from memory and never exist at all. I tend to write down topics, snippets of dialogue, ideas for plot lines, but how do you capture the essence of an image without creating it?