I wrote a simple web service (in 2 minutes - excuse the non-optimal code) that wraps the webshot.exe for use on Windows based servers (http://www.websitescreenshots.com/). You must have webshot.exe installed.

You submit a URL and it returns a link to the captured .png file stored locally on your server.

[WebMethod]
public string HTML2PNG(string URL)
{
string result = "";
string filename = System.Guid.NewGuid().ToString() + ".png";
string destinationURL = "http://your_ip/html2png/"+filename;
string outputPath = "C:Inetpubwwwroothtml2png";
filename = outputPath+filename;

if(URL != "")
{
try
{
string cmdStr = "/url ""+URL+"" /bwidth 800 /bheight 600 /quality 65 /encoder png /out "+filename;

Process myProc;
myProc = Process.Start("C:ProgramFilesWebShotwebshotcmd.exe",cmdStr);
myProc.CloseMainWindow();

result = destinationURL;
}
catch(Exception ex)
{
result = ex.ToString();
}
}

return result;
}