Insted of double Quotes use singleQuotes
<% if(Eval("img_url") != "")
{%>
<img alt='<%#Eval("name")%>' title='<%#Eval("name")%>' src='../img/avatars/<%#Eval("avatar_url")%>' style="width: 80px; height: 80px">
<%}
else
{%>
<img alt='<%#Eval("name")%>' title='<%#Eval("name")%>' src="../img/blank.gif" style="width: 80px; height: 80px">
<%} %>