magento how add dynamic url path in system.xml

I am working on payment gateway. but i have problem i did google research but couldn't find any solution i hope i'll get solution here so i am posting here

following my system.xml code block

<title translate="label">
    <label>Title</label>
    <comment><![CDATA[<img src="/media/billmate/images/billmate_bank_s.png" />]]></comment>
    <frontend_type>text</frontend_type>
    <sort_order>3</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</title>

in this block problem is in comment tag right now here i put static link /media/billmate/images/billmate_bank_s.png please anybody suggess me how to make it dynamic

An element from system.xml can have a dynamic comment. The comment can be rendered through a model.
You need to declare the comment field like this:

<comment>
    <model>module/adminhtml_comment</model>
</comment>

Now you need to create the model with alias module/adminhtml_comment:

<?php
class Namespace_Module_Model_Adminhtml_Comment{
    public function getCommentText(){ //this method must exits. It returns the text for the comment
        return "Some text here";
    }
}

like following

<title translate="label">
    <label>Title</label>
    <comment>
        <model>module/adminhtml_comment</model>
    </comment>
    <frontend_type>text</frontend_type>
    <sort_order>3</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</title>

return value from the getCommentText method