Hi Derek,
Thank you for the answer.
We actually don't use esri basemap but our local detailed basesmaps
I see also that basemap credits are maneged by the informatin specified in the credit field in the webmap setting (i.e Openstreetmap basemap) that appear like on-screen text in the mapviewer bottom.
I think that the logo on map viewer shoud be a inportant customization options, to brand correctly the Portal application. The same thing is possible in any web application customization option.
Enrico