Hey Shiu,
I was literally just preparing a stripped down archive to send to you via email, removing images and files that were not needed to show the issue, and inadvertently found out what the problem is.
In the folder where "mouth.png" is (along with all the other standard character images), I also have a folder called "mouth" which contains all the other variations of mouth images for the character.
By deleting this folder to strip down the example to send to you, I did one last test and it worked as I would expect it to - perfectly!
I then restored everything to the way it was and the problem appeared again - this is where the penny dropped and I realised the folder name was the same as the image, apart from the .png extension.
So it seems it is the fact that I have a folder which has the same name (minus extension) as the image is causing the problem, as Spine must think the path is pointing to the folder called "mouth" and not the image I am dragging in called "mouth.png".
This is why renaming the image fixed the problem, but also renaming the folder has the same effect, as long as they are not the same.
Strangely Spine doesn't get confused when showing the folder and image in the Tree View, only when it is dragged into the editor.
Not sure if this is a bug or a "gotcha" kind of feature, but I have wasted a couple of hours on this unfortunately so it is definately something which if it is not a bug, should be at least mentioned in some kind of FAQ or Usage Guide document.
It should be easy to reproduce the setup your end with 2 images and a folder, but I can still send a sample project if you like?