Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim file As String = GetRndFile(New IO.DirectoryInfo( _
        "C:\Fotos")).FullName
        PictureBox1.Image = System.Drawing.Image.FromFile(file)
    End Sub
 
    Private Function GetRndFile(ByVal dir As IO.DirectoryInfo) As IO.FileInfo
        Dim rnd As New Random
        Dim fil As New List(Of IO.FileInfo)
        fil.AddRange(dir.GetFiles)
        Return fil.Item(rnd.Next(0, fil.Count))
    End Function
 
End Class