Trying to copy from outside the bounds of the src image may have introduced the black, just a guess

If that was just a guess, it was a very good guess...
Although I suspect it was an educated guess at the very least :)

Either way, that has worked!
Thank you very much huck.

Because the problem was only appearing with a real picture as the image and not a pattern created in GD, it seemed to be a palette issue. The effort had been looking there rather than the way the foreground image is placed on the background...