The auto creation of map thumbnails in ArcGIS online is inconsistent and also removed from user control.
It is inconsistent because layers in the map sometimes don't appear in the auto created thumbnail image, and the saved extent of a map sometimes isn't reflected in the thumbnail extent.
The user should have more control over the auto creation of a thumbnail from the map image. There should be a tool that will guide the thumbnail creation process. Often even the saved map does not make a good thumbnail and one wants to show a small part of the map to highlight their symbology.
You shouldn't have to break out GIMP to do it.
Obviously if the thumbnail is a picture of a firetruck or duck, and is not being made from the map then you still also should have the option to upload your custom graphic as you do now.