When you publish a hosted feature layer to ArcGIS Online there should be a setting to choose a custom image for the feature services' icon inside the service editor. (see attached image for idea on placement, etc.)
The reason I am requesting this is because we have several hosted feature layers on ArcGIS Online that we share with the public. We built a gallery for data downloads and the hosted feature layers have custom images uploaded for the icon to make for a cleaner look in our gallery (see attached icon image for example). The only downside to the custom image for these hosted feature layers in that each time we update the hosted feature layers using a python script, the custom image is removed and is defaulted to a snapshot of the feature layer geometry.