I think only the artist can offer a collection to the character owner... at least that's what I figured idk I keep forgetting about it :U

When you submit an image, if you go back to it's submitted page as the one who submitted it, you can offer to a specific user to add said image to a collection, on request by them or by offering in general such as for commissions, or what ever other reason you choose to. On the right side under the image, above the Tags, you can enter a username to offer the image to as a collection image. It shares the file into that persons collection gallery however it is still you that submitted it. Kind of like how an external link works, but to every picture in the collection gallery.