I just recently had a similar problem
. And I also think Sproingle(sp?)'s new FAQ thread covered this (can't find the link).
The solution that worked for me was answered in the above thread with
URL url = this.getClass().getClassLoader().getResource("resources/Ball.png");
The one in sprongles FAQ sounded like it was more reliable, it was something along these lines:
Oh god, it was nothing like that, but it started with Thread. :L
*searches for spoingle faq*