use simple BBCode in your comments 
atm follow tags available:
[b]bold[/b] 
[i]italic[/i] 
[u]underline[/u] 
[color=color]text[/color] example [color=FF6600]Text[/color]
list 
[ul] 
[li]number 1[/li] 
[li]number 2[/li] 
[li]number 3[/li] 
[/ul]
[url=http://www.google.com]Google[/url] 
[img=http://uri.to.pic][/img] (fix picsize)
(required wp-syntax) 
[code]your code[/code], 
optional with declaration of the language 
[code lang=”csharp”]your sample code[/code] (all available languages from wp-syntax possible)
<h3>Arbitrary section</h3>
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BBComments

utdemir

With this plugin, you will get two links; One link for the image, and one link for the redirect page. Then you can use these links in a forum signature like this:
[url=example.com/wp-content/plugins/p2i/1.php][img]example.com/wp-content/plugins/p2i/1.png[/img][/url]
It also supports shortening the url’s with “is.gd”.

WP-Posts to Image Plugin

Let's you show your blog's last posts on a forum signature or stg. uses bbcode.

BBComments vs WP-Posts to Image Plugin