How can I use predefined values (default values) for web content structure Image fields

Issue

I am trying to set and use the Predefined Value for an Image field I added to a web content structure, when configuring the field (on the left side)
However, I am not sure what exactly I should write in the Predefined Value (what format, syntax) and whether it works after that. It looks not to be working, actually.

Indeed, for Image fields that setting will not work this way (this is being discussed on some LPS tickets)
But there is another (and currently better) way of setting the predefined values for the fields of a web content structure. We are talking about the "Edit Default Values" feature, which you can reach after having created a structure. So the steps would be something like this:

1. Create the structure with an Image field and save it. (you don't need to set predefined values at this point, so just leave that part out)
2. After you saved the structure, list the structures and click the 3-point icon next to the desired structure and choose "Edit Default Values": you will be presented with a screen where you can properly fill in the default (predefined) values. You can normally select an image here. Save.
3. Start creating a web content from this structure: the rendering is fine and you see those default values.