Also, if you not bothered about people adding multiple words in different
cases (Aka: Blah, blah, bLaH, etc) then it can be shortened to one line,
like:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
   If Not ComboBox1.Items.Contains(ComboBox1.Text) Then
ComboBox1.Items.Add(ComboBox1.Text)
End Sub

Example of a custom class overriding the .tostring function
(overriding isnt something you will probably need, but...)
(if you get confused... dont worry - it doesnt look like you needed custom
classes anyway)

Description:
Button1 puts the custom object into the combobox (and uses its .tostrong
override), Button2 copies the selected-object from the combobox to the
picturebox... this could be any of the objects in its list, although the
example only shows adding one object derived from the custom class.  The
objects in the combobox could be a mixture of different custom objects,
strings, images, other comboboxes, etc.

Controls needed on form:
*ComboBox1
* Button1
* Button2
* Picturebox1

Code:
Public Class Form1
   Private myImg As New cMyImg("C:\smiley.jpg")

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
       ComboBox1.Items.Add(myImg)
   End Sub

   Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button2.Click
       PictureBox1.Image = ComboBox1.SelectedItem
   End Sub
End Class

Public Class cMyImg
   Public Img As Bitmap
   Private ImagePath As String = ""

   'Here is an overriding function
   Overrides Function ToString() As String
       If ImagePath = "" Then
           ToString = "{NoImageSet}"
       Else
           ToString = "{" & ImagePath & ")"
       End If
   End Function

   Public Sub SetImage(ByVal filepath As String)
       Try
           Img = New Bitmap(filepath)
           ImagePath = filepath
       Catch
       End Try
   End Sub

   Public Sub New(ByVal filepath As String)
       Try
           Img = New Bitmap(filepath)
           ImagePath = filepath
       Catch
       End Try
   End Sub
End Class