Any images that are, approximately, more than 1 or 2 month/s old and saved in your browser, will not be able to be un-favorited. When you click on the image, the system will show you the other 3 images (total 4) and all will have the same option to make favorite by clicking on the star (but not the opposite). If you have a ton of images your only option will be to, maybe, clear the cache and this will probably delete everything? ... it will take me a very long time to test this after I manually save everything and record related information.
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