I would suggest you look at
regular expressions[
^].
A quick C# solution (maybe not the best as my RegExp is not as good as it should be):
string main = "kgjhnbkfm glnvfkdşlv http://ghkng.xfg/ngbf.png gvlkdjg";
Regex pattern = new Regex("(.*)http://(?<urlfinal>(.*)).png(.*)");
Match match = pattern.Match(main);
string urlFinal = "http://" + match.Groups["urlfinal"].Value + ".png";
</urlfinal>
It should be easily convertible to VB.NET.