Render an image that you want to be the thumbnail, in a 91x91 window. Export image directly to the relevant folder as "MATposefilename" and type .png. Or, you can render it large, export it however you like and postwork it, then save it as a .png at 91x91 in the correct folder and filename. Hope that helps :)

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire