Hey Fred,
while I don't know the program either I'm trying to teach myself the same thing.
I'd sort of recommend the opposite, take your pics against a white background so you can mask them out easier. The slight fuzzy white outline can be trimmed as most programs that allow you to mask allow you to shrink the mask by a pixel (or set amount).
The other thing to do is to either include in the mask the areas that show through white such as behind the wheel spokes so that they are transparent and allow the background through or to airbrush them out in the paint program later. I use a combo of both.
Finally I'd suggest fiddling with the light/contrast brightness of the image to match the background. I tend to darken and soften the highlights, plus you can always airbrush in a filter on some areas ....
cheers
Brent
Heres a few I've done ....


