What is your primary reason for wanting to use an external list in this case? How many choices do you have?
We don't currently really support images this way either because Central does not have a way of attaching arbitrary media files to a form. You can see how folks like @ahblake overcome this limitation at Pull data with photos - #6 by ahblake
What we'd like to do longer term is have a concept of media attached to entities. That will make it possible to keep the media in sync with their corresponding entities rather than having a CSV and media that need to be kept in sync by some external process. We don't have a timeline for this yet, though. As @Grzesiek2010 states, we maintain search() for existing forms but don't document it or expect to build further on it.