How to allow users upload images to existing nodes?
The best / simpler way would be to create a second content type called 'image', and give user the permission to create nodes of type 'image'. You can add a CCK imagefield to provide an image upload field.

Then you will want to be able to store relationships between your movie node and the related images nodes. You can add a node reference field on the image content type. You can populate this field by default with the movie reference. So any image added in the website will be related to a movie.

You can then create a view that will list the images related to your movie ID.