I'm having what appears to be a common problem with multi-site installations: broken images. I upload an image to subdomain.domain.com and the image appears broken (just a question mark in a blue box.) I upload the same image to domain.com, which is site 1, and the image displays normally.
I'm using subdomains. I've also read .htaccess could be involved. What is the best multi-site .htaccess and how do I go about debugging this problem?