The ArcGIS Online home page banner image does not support transparency.
Last Published: October 29, 2014ArcGIS Online
Bug ID Number
September 29, 2014
March 30, 2022
Operating System Version
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
This is by design. The container for the banner image has a background of white and a drop shadow by default