Complete solution with regex :
string source ="<img src=\"http://MyUrl.JPG.jpg\"";
var reg = new Regex("src=(?:\"|\')?(?<imgSrc>[^>]*[^/].(?:jpg|bmp|gif|png))(?:\"|\')?");
var match=reg.Match(source);
if(match.Success)
{
var encod = match.Groups["imgSrc"].Value;
}
solved Extract image URL in a string (RSS with syndicationfeed)