[{image:{imageUrl:"http://imageurlsite.com/xyz"},additionalImages:[{imageUrl:"http://imageurlsite.com/pdq"},{imageUrl:"http://imageurlsite.com/lol"},{imageUrl:"http://imageurlsite.com/zomg"}]}]

db.collection.aggregate([{$group:{_id:null,image:{$addToSet:"$image.imageUrl"},additionalImages:{$addToSet:"$additionalImages.imageUrl"}}},{$set:{images:{"$concatArrays":[{"$first":"$additionalImages"},"$image"]}}},{"$project":{images:1}}])