With Scrum / user story / agile development, how does one handle scheduling out-of-sync tasks that are part of a user story?

We are a small gaming company working with a few remote consultants who do graphics and audio work. Typically, graphics work should be done at least a week (sometimes 2 weeks) in advance of the code so that it's ready for integration. However, since SCRUM is supposed to focus on user stories, how should I split the stories across iteration so that they still follow the user story model? Ideally, a user story should be completed by all the team members in the same iteration, I feel that splitting them in any way violates the core principle of user story driven development.

Also, one front end developer can work at 2X pace of backend developers. However, that throws the scheduling out of sync as well because he is either constantly ahead of them or what we have done is to have him work on tasks that not specific to this iteration just to keep busy. Either way, it's the same issue as above, splitting up user story tasks.

If someone can recommend an active Google agile development group that discusses these and other issues, that'll be great. Also, if you know of a free alternative to Pivotal Labs, let me know as well. I'm looking now at Agilo.