You need to render the generated 3D model, scale it down to 32 pixels height, then clip the alpha (in GIMP this would be Layer -> Transparency -> Threshold Alpha...) to create sharp outlines.
Personally, I render them in 3D software, but you could also simply take screenshots of the 3D preview on the TRELLIS website from each side, and remove the background with a magic wand tool.
I used the simple technique of taking screenshots for this example, and I think it looks quite good.
