I'm no artist, apart from maybe stick figures.
A basic scene may need to be agreed on and then if it'll be just a collage, or a scene with certain folks doing certain parts or elements as well as certain characters. One thing that may make things easier if if a universal type of file is used that can be used and saved to by whatever programs the various artist are using. As how to send the image to each it can be done by msn, or by email. Though I think each person in the network should keep separately the version sent to them and their work, in case there is some mess up somewhere, then it'll be far easier to fix.