When you mentioned the white version, I HAD to see how it would look, and got pretty much what you have there, which definitely didn't do it for me the way the original did - but as they say, the customer is always right, even if they're wrong... when you know your target size, bounce it through to me at auxotic at gmail.com and I'll upsize it for you...
Did you select it with the magic wand?? Have you tried adding a few more pixels to keep a black, feathered edge in the image?? I've tried a few things on selections I've had to 'move' from black to white backgrounds, and one of the other successful 'blends has been to make a duplicate of the original, used levels to make the duplicate black, given it a guassian blur to add soft edges, and placed it UNDER the original selection - you can change the opacity to suit, but the soft under shadow helps blend the previous black edged image to a white background...
Cheers,
Stuart