icon???Applicable for basemap layers. Specifies the location of the icon displayed for this layer in the basemap gallery. For example, the "Streets" ArcGIS Online basemap icon's property reads as:https://www.arcgis.com/sharing/rest/content/items/d8855ee4d3d74413babfb0f41203b168/info/thumbnail/tempstreets.jpg
I still don't see why they would have exclude operational layers in the BaseMapSwitcher widget config file but I'm not going to get bogged down by details ....
Well the reason that your streetmap that you manually specify is missing the thumbnail preview is the fact that you are not specifying the icon attribute of the basemaps layer tag ... Hope this helps.
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