My process:
I create a new project folder in the Spine root folder where I plan to store all my images / project file.
I copy relevant image files from Dropbox into this project folder.
In spine, I create a new project. I see the new project folder and the images I created in the Spine hierarchy. The images can now be added to the scene, moved around, manipulated, etc.
I then save the project into the same project folder as the images, and immediately, all the images in the scene show as a "missing" graphic.
Maybe I've missed something fundamental, here.
Thanks for any help!
