First, from the rules:
5) Please ask the creator for permission to use their images before using them. Anyone not doing this will be BANNED.
You MUST have permission from the creator of an image before using it. This includes use on your own site or use in a document on a site of your own. It also includes using an image on someone else's site. Simply giving credit is not sufficient if permission has not been explicitly given by the artist.
The only time explicit permission does not have to be given is when the image is in the Free Use Forum. If the image is there, it is understood permission has been given. But do remember to credit the artist.
If an image is in the Requests forum, the only person who has permission to use it is the one who made the request. If you want to use an image there, contact the artist by PM.
If anyone has any questions about this, or if any part of it isn't clear, PM Michael or Quo. If you want to discuss it in a public setting, feel free to start a thread in General Discussion.