It's becasue the font you are using doesn't have a glyph for that character. You'll notice that it does show in other places, where a different font is used. The only way to fix it is to use a font that does have a glyph for that character (if you do that, you might not have the same problem with some other character), or use not use special symbols.
I've noticed that I get this more in Messenger than anywhere else, I don't know it it's because Messenger doesn't handle the extend characters well, or just because people on my list use crazy Display Names and I just don't encounter those characters in any other program.
PS
Use thumbnail images!
Like this:
code:
[url=http://img217.imageshack.us/my.php?image=bvn5.jpg][img]http://img217.imageshack.us/img217/2883/bvn5.th.jpg[/img][/url]