I moved in new functionalities, because it could be awesome.
But still if somebody has an idea about it can post here an answer.
For my part, I would say a simple normal default string pointing to a valid image path.
Is it a stupid question ? or a stupid answer ?
Maybe I did not got it...
I think you can also test it in sandbox :
1. Make an upload of your default image in your component (play with the rename property to name it as your please)
2. You can delete the item that you have used for that upload, but be careful it does not delete the image. (must unckeck this option in the image field type)
3. Enter your image path in the default value of the image field in the builder. (you can also try 'prefills form')
4, That's all.
It uses indirect files accesses ex : [DIR_XXXX_XXXX]/default.png
It uses the thumb engine.
Try it... It should work.