Restricting external images in signatures and avatars

[ProfC]

Hi there.

My forum has concerns about people attempting to IP harvest using 1x1 clear images hosted on remote servers (some people take NationStates far too seriously), and while it hasn't happened to us yet, I'd like to prevent it from being possible.

We are already using the Import External Images product has worked to greatly eliminate the potential for this, as has MarkFLs Disable BBCode/Smilies per Usergroup/User, but we don't have anything currently which (to my knowledge) handles avatars, profile pictures and signatures.

Ideally, and the best case scenario would be to allow a user to reference an internally hosted image (such as in the Albums or filedirectory) or an external image from a whitelist of sites, like Imgur or Photobucket. Or failing that, does the same job as the IEI above.

I'd appreciate it if someone would be kind enough to point me in the right direction in achieving this.

Thank you in advance!

~ProfC