I played around the file you posted at the time and tried those different transparency settings but still could not really get hang of it. Maybe it is just because never really have used those for things like that. (Or most likely I just suck....) Artworks has mix, Stained glass and Bleach transparency types but never really did anything with those.

The car model I used is nice but I can not take any credit for it as I only rendered it with alpha channel because it suited for the purpose. People are free to use it to play around as the model can be downloaded and used freely.

Software I used for the image is not actually for bitmap editing but more like combining and manipulating images that already exist. User interface is very simple but effective and even a child would not have any difficulties using it. There are no layers, just images you drop into canvas and those images can be manipulated with different masks and tools. Best of all it will also directly render Draw/Artworks vector images including automasking etc so will work great as suppporting application. Too bad there is nothing similar on PC as it actually is excellent and I still use it regularly, even if people have lot more poweful mobile phones than the system it runs on.