Go into your postbit template and find:
<a href='Warn.php?do=WarnUser&id=$post[userid]&post=$post[postid]'>Warn $post[username]</a>
Replace that with:
<a href='Warn.php?do=WarnUser&id=$post[userid]&post=$post[postid]'><img src='images/misc/whatever.gif' border='0'></a>
That should replace your textual link for "Warn XYZ" with the image whatever.gif, which is found in your images/misc directory.
Do the same for the other two links ("View XYZ's warnings" and "View your warnings") using a suitable image.
The code for the other two links is:
<a href='Warn.php?do=ViewWarnings&id=$post[userid]'>View $post[username]'s Warnings</a>
and
<a href='Warn.php?do=ViewMyWarnings'>View your Warnings</a>
Then use "Textual Links" option, from your Warning Options.
Rgds