Dim webClient As New System.Net.WebClient

Dim url As String = "yourPageURL"
Dim htmlByte() As Byte
htmlByte = webClient.DownloadData(url)

Dim htmlString As String
htmlString = System.Encoding.UTF8.GetText(htmlByte)

' Generally using Regexes for Parsing HTML is not a good idea, this is just an example, you would use something like HTMLAgilityPack for parsing.

Dim Matches() As MatchCollection = System.Text.RegularExpressions.Regex.Matches(htmlString, "<img.*?src\s*=\s*[""'](?<src>[^""']+)[""'].*?>", System.Text.RegularExpressions.Regex.RegexOptions.IgnoreCase);

For Each match As Match In Matches
    If (Not match.Groups("src").ToString().ToLower().Contains("captcha")) Then Continue For

    PictureBox1.Image = New Drawing.Bitmap(New System.IO.MemoryStream(webClient.DownloadData(match.Groups("src").ToString())))
Next