Okay.. I think I should.
I've used Paint.NET to do this. First, open up your car in Zmodeler. and press PrtScrn to take a screenshot. Then open Paint.NET and press Ctrl + V to paste it there.
Next, from the top view, roughly select the car using rectangular selection tool:
Press Ctrl + N to create a new file in Paint.NET, and paste this selection there.
Now use the magic wand tool and select the car body. you'll need to set the 'tolerance' value of the tool as per requirement. Select paint bucket tool and paint the selection with black colour, there will be some white patches in between, make them black by using brush tool.
Now again, use the magic wand tool to select this black region, this time, it would select much more accurately because its all single coloured. Copy this selection.
Now make a new image in Paint.NET of dimensions 256x512 pixels, it would be white, use the eraser tool to make it transparent.
Make a new layer from the layers window and
paste the selection on this new layer. At this moment, Paint.NET may say the image being pasted is bigger than the canvas size and ask to increase the canvas size. Don't increase the canvas size, adjust the copied shadow so as it fits inside the rectangular canvas perfectly. It should look like this:
Now reduce opacity of this layer to a value of around 180:
Press Ctrl+Shift+F to merge the 2 layers into one and save as 32-bit .TGA and you're done. Hope that helps. Car used: EnzoSkylineMuira's VW Golf.