I'd like to suggest two ideas: you could either have the star/heart in one corner of each thumbnail, clicking it will toggle whether it's in the favorites list or not, and this would be indicated by having an outline and a solid version of the icon.
Edit: for clarification, the space occupied by the heart/star icon would toggle the favorites membership of the station, the remaining space of the thumbnail will have the current behavior (starting playback).
Another, IMO less intuitive way of doing this are context menus that pop up and allow adding/removing the station to/from favorites. You might have seen something similar on Android for instance.