I am using the Enhanced Basemap Widget with Web AppBuilder Developer Edition 2.0 in Firefox (also testing in IE 10)
It works good, except that the basemap thumbnail is not visible when I launch the app.
I have created jpegs (18kb and 13kb), 400x400 as that was what I prompted to make. The jpegs are saved under:
I have attached a screenshot of what I am seeing after launch.