I've created an issue:
The reason Spine only detects the visible images is because it doesn't know the others have changed size. To know that, it would have to load every image. There could be thousands, so that's not something it can do periodically just to check. We'll need to provide a button to scan all the image sizes.
Maybe the best available solution at the moment is to make each skin visible and answer the popup. use hotkeys where possible, eg
Next Skin. I apologize this may still be tedious. It may be easy to miss a mesh that is only visible in an animation. 🙁
To see if
Keep size and
Stretch are working correctly, look at the images in edit mesh mode with
Deformed unchecked. We'll also check it's functionality.