Added duet image

_data/features.json

@@ -10,7 +10,7 @@
 },
 {
 	"title": "Duets",
-	"image": "",
+	"image": "duet.png",
 	"description": "Sing real duets, with different notes for each player. Players can select which part they want to sing, independent of the microphone they are assigned to."
 },
 {