Create list of unique values from list of objects

My data (PD1Client) contains a list of objects with multiple properties. I am attempting to assign an app variable with a list of unique property values from one specific property (“GolferImage”; which is an image URL type). I defined the app variable as a “list of image URLs”. I have attempted to set the app variable with the following formula, but it doesn’t work:

PICK_KEYS(UNIQUE_BY_KEY(SELECT(data.PD1Client,!IS_EMPTY(item.Player)), “GolferImage”),“GolferImage”)