You can change the icon image for playables in Playground easily, just follow these steps:
Select playable in playground
Click "App Settings" on the left of the page
Click on the enlarged image you wish to change
A window will open allowing you to select a new image for the app icon
These icons are only used in Playground and are not exposed in exported builds or elsewhere. Changing the icon does not impact your Unity project.
If you wish to change the icon displayed whilst your playable loads please click here for more information.