Image hosting site Photobucket has recently made changes to their service, requiring payment for the ability for images hosted there to be viewed on external websites (such as serebiiforums).

You can read more about the changes here.

Please check your signature and profile page images, and replace any images that are hosted on Photobucket. Even if the image shows up normally at the moment, it will likely be turned into the above image in due course.

Imgur is an alternative.