string str = @"<img src=""http://img.cn.com/a/latest.gif"" /> <img src=""http://img.cn.com/topics/latest.gif"" /> <img src=""http://img.cn.com/b/free/latest.gif"" /> <img src=""http://img.cn.com/topics/latest.gif"" /> <img src=""http://img.cn.com/main/c/latest.gif"" />"; 
Regex reg = new Regex(@"(?is)<img[^>]*?src=(['""\s]?)((?:(?!topics)[^'""\s])*)\1[^>]*?>");
foreach (Match m in reg.Matches(str)) 
{
     Response.Write(m.Groups[2].Value + "<br/>"); 
}

/* 
http://img.cn.com/a/latest.gif 
http://img.cn.com/b/free/latest.gif
http://img.cn.com/main/c/latest.gif 
*/