To do so, you would have to edit the templates !
You could try that :
Removing the alt class from the mgc_chatbox_bit and then editing the mgc_chatbox template by adding style="css code here to add a background image" in the table tag of the following part of the template :
Code:
<table width="100%" cellpadding="0" cellspacing="2">
<tr>
<if condition="$can_use_team">