Images (.jpeg, .png, etc) added to ArcGIS Online as items fail to display the thumbnail.
Last Published: June 23, 2015ArcGIS Online
Bug ID Number
April 30, 2015
August 21, 2020
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 behavior is by design.
Go to the item details page of the image > Edit > Add the same image as a thumbnail image. The Search window and Group details window also display the thumbnail image.