You may need to check the "Child types" setting.

Go to "CMSSite Manager" -> "Development" -> "Document Type" -> "Root" -> "Child types".

For each document type, you can set which particular document types could be added as child. If you want to add your new "Image" document type under "Root", you need to select your the "Image" doc type from the list as child for "Root". This would allow you to add the "Image" document under Root now.

Change this setting again on other document types if you want to add your "Image" on different document type such as "Folder" and ...

Cheers,
Matt