Note that these assets should be game ready, meaning that the polycount should be reasonable to use in a Mona space. The number of Materials should also be low as each new material increases draw calls.
Asset Preview Image
It would be preferred to create the image similar to the assets in the Library. This will improve the overall experience for browsers of assets. There are several ways to create this however, and you can use whichever method suits you.
Examples Asset Images
Use the Asset inspector window by enlarging the window as big as possible, then taking a screen shot using Print Screen (or Alt + Print Screen for just the Unity application window). Use your favourite Image Editor to save the image. Most assets so far are using a 750x750 size image.
Screengrab the Inspector
2. Render the asset in your favourite asset creation tool. Make sure that the background uses a similar grey to other assets in the library. The grey in question is #313131, or R: 49, G: 49, B: 49. It is best if the asset is facing the bottom left.