The structure is really up to you.
But if you’re using multiple worlds, it might be prudent to store your shareable assets outside of your world folder.
So if your local folder structure is like this:
Instead of adding images to Data/worlds/world/scenes/whatever, you could just add a folder at Data/assets/scenes/whatever.
Then when you use the importer for the Forge and select your Data folder, The Forge will be able to grab the images easily.
There have been issues with this where various users perhaps use this method or just files from another world and only import the world, meaning The Forge cannot see the files, as they were never uploaded, so it won’t work.
But uploading the entire Data folder will work perfectly.