I’ve seen one other request for a Default Value for Image types but it was because they needed it for a new type they were adding.
I’m requesting this feature in general. I don’t see why an Image is any different than other field types in that there can be a default when a new Post is created. Perhaps a default would not be possible when choosing to use an Image Object. But for Image ID and Image URL, there should be a default option so that I can enter the ID or URL of a previous uploaded image.
For instance, if I make an ACF for a banner image, it would be nice to be able to upload a banner image meant for a default banner, and then use its attachment ID as the default for my field. This makes it so then when a user creates a new post, the default banner is already visible to them and gives them a better idea about if they want to change it or not. It also gives them the option to remove it and have no banner image.
If I were to just use the template code to show a default image when no image is selected, it eliminates the option of not showing an image.