I have been trying to add some additional base maps to a basemen widget in ArcGIS Online and I have found several that appear not to work and have come to the conclusion that it is related to the basemaps being image services? Is this true? For Example I cannot get widget to recognize the following url as a valid basemap; http://gis.apfo.usda.gov/arcgis/rest/services/NAIP/Oregon_2014_1m/ImageServer
I just added your imagery service layer in WAB 1.3 to the basemap gallery widget and it worked fine.
EDIT: Looking at the code in WAB 1.2 it should support image service baemaps as well.