Do you mean how to make a thumbnail that will make people click your image or simply "how do I make a 200x200 pixel picture?

The first.. takes some skill and luck. I usually make a new pic 200x200 pixels in Photoshop and use a copy of the render which I move around until I find a suitable part for a thumbnail. sometimes it's easy, sometimes... not so much.

And that partially answers the other option, the how (practically): I use Photoshop. You can use any image processing software.

A third option is to do a render 200x200 pixels. either of the full scene if it fits the aspcect ratio OR a part of the scene. 

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.