Well there is no instant message system on this forum, it's been looked into before and occasionally brought up in the Admin section. There is this and there is the ability to put instant messaging services that you might be already a part of in your info and it would show and you would presumably be able to communicate instantly that way - if any of us actually used the instant messaging like icq, yahoo or any of the others.
As for using HTML in your sig - that's what I do, my artwork sig is hosted elsewhere and HTML connected up. You can 'attach' images to your post and they will show up at the bottom of your post. The size limits are given when you pull up the 'manage attachments' menu and from there you can attach various sorts of files. Some folks creatively use HTML to attach bigger pictures and will use photo sites from which to host their pictures. Usually if someone wants to share a mp3 or video they'll have them hosted elsewhere and linked where we'll go view and listen to them. The software for the forum is vbulletin, it's been recently updated for the site and I think everyone is still learning it but it's not a version that allows video embedding and such. This has been a laid back forum and previous to the religious discussion the biggest dust up here had to do with girls in bikinis with guitars so staying calm and mature and no name calling and you know, that sort of thing. Anyone with 'Swat Team' in their name can help you, we're the admin folks, Red is like a Super Admin and Brad W is the head guy. That might about cover it.