Are you talking about a copyright mark? That's not a "tag," or at least, not what we're meaning as tags. A tag is a URL to another fan site, or someone's live journal name, for example.
If it's from a valid media site, like CNN, and they put a mark, it's okay. The same goes for a modeling agency's public profiles. But you should provide a link in that case.
Places like Getty Images put watermarks on their pictures - they are okay to post because they are a publicly available press image.
If you've got any specifics I didn't answer, send me a PM.