Код:

<script type="text/javascript">
defaultPict = "https://forumupload.ru/uploads/0000/12/13/61-1.png"

var arr=document.getElementsByTagName("ul")
i=1
while(arr[++i]){
str=arr[i].innerHTML
if(str.indexOf("pa-avatar")==-1) {
q=str.toLowerCase()
if((pos=q.indexOf("<li class=pa-from>"))!=-1 || (pos=q.indexOf("<li class=\"pa-from\">"))!=-1 || (pos=q.indexOf("<li class=pa-reg>"))!=-1 || (pos=q.indexOf("<li class=\"pa-reg\">"))!=-1){
nstr =str.substring(0,pos)
nstr+="<li class=\"pa-avatar item2\"><img src="+ defaultPict +" alt=\"Аватар\" /></li>"
nstr+=str.substring(pos)
arr[i].innerHTML=nstr
}}}
</script>