I would suggest not adding it in the css unless you want a different css for every member and an if checking for it.
I suggest in the userinfopane find this
<div itemscope itemtype="http://schema.org/Person" class='user_details'>
change it to this with as many if statement as you want for the groups you want custom backgrounds.
<div itemscope itemtype="http://schema.org/Person" class='user_details' style='width: 155px;
float: left;
font-size: 12px;
text-align: center;
<if test="$author['member_group_id']==4">background: url(http://www.katagon.com/forums/public/style_extra/userbg/fmod_bg.png)</if>
<if test="$author['member_group_id']==6">background: url(http://webflake.sx/uploads/profile/photo-29622.png)</if>
no-repeat; min-height: 394px;'>