Transitions are between the same item (or "instance" to be fancy) when it has different properties (like position, size, opacity, rotation,etc) in different states. When you import an image, you create a new item in the Art Board. If you copy-paste, you create another item.
If you want the same item to show up in multiple states, right click it and choose "Share to State" and the state you want it to appear in.
There's also a "Make same in All States" that makes an item have the same appearance in all states that it's present in.
For your specific case, though, you have transitions already working on an image, so you should just change the image to have a new source. Click the image that had been working before, click the "source" in the Properties Inspector (lower right), and you should have the ability to change where your image is coming from.
Found it myself (finally)
States/ Share to State.