Smooth thumbnails?

[Garamonde]

Well people usually render their models in their modeling programs like Blender and 3DS Max and so on.
The easier way to do it though is to just make sure that anti-aliasing or smooth resize is turned on when you shrink your model images. I take screenshots of the model zoomed in and then downsize them to the exact dimensions of the icons and previews (95 x 79 and 354 x 353 respectively) to make them nice and smooth like this:



Of course you don't have to follow the exact dimensions if you don't want the model touching the edges of the icon/preview, I just do that so it shows as much of the model and as big as possible.

I also wrote about this is my model ripping guide but this is the basis of what you need to know regarding making decent icons/previews.