Do you mean getting thumbnail of a webMap for which you have an URL in this format: http://www.arcgis.com/home/item.html?id=8bf7167d20924cbf8e25e7b11c7c502c ?
You get always get a thumbnail from the portal item if it exists. Once you have you portal item initialised you can call thumbnail and it will return an image if it exists.
If you're asking how to create your own image/thumbnail for portal item.
You can load the webMap and once it's rendered you can call exportImage on mapView.