The following code only displays one thumbnail rather than the multiple ones selected. Can anyone please explain why?

the edits below should create a vertical list of images. if you want columns and rows, it will take a bit more math work. I think the easiest way to go would be the listview. Using the edited code below will at least help us determine if the imageControl's images are different.

Public Class Form1

  Private Sub Browse_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Browse.Click
    Dim OpenFileDialog1 As New OpenFileDialog
  
  'added code
    Dim curTop As Integer = 0

    OpenFileDialog1.Filter = "Images (*.BMP;*.JPG;*.GIF,*.PNG,*.TIFF)|*.BMP;*.JPG;*.GIF;*.PNG;*.TIFF|" + "All files (*.*)|*.*"
    OpenFileDialog1.Multiselect = True
    OpenFileDialog1.Title = "Select Photos"

    If OpenFileDialog1.ShowDialog() = DialogResult.OK Then

      For Each file As String In OpenFileDialog1.FileNames
        Dim imgFilename As String = IO.Path.GetFileNameWithoutExtension(file)
        Dim img As New System.Drawing.Bitmap(file)
        Dim imgImage As Image = Image.FromFile(file)
        Dim imageControl As New PictureBox()

        imageControl.Height = 100
        imageControl.Width = 100
        Dim myCallback As New Image.GetThumbnailImageAbort(AddressOf ThumbnailCallback)
        Dim myBitmap As New Bitmap(file)
        Dim myThumbnail As Image = myBitmap.GetThumbnailImage(96, 96, myCallback, IntPtr.Zero)
        imageControl.Image = myThumbnail
        imageControl.Top = curTop 'code added
        curTop += imageControl.Height 'code added
        PhotoGallery.Controls.Add(imageControl)
      Next
    End If
  End Sub

  Public Function ThumbnailCallback() As Boolean
    Return False
  End Function

End Class

Also on a note, Change PhotoGallery from a Picturebox to a Panel Control So that you can scroll through your thumbnails. Just Set the AutoScroll Property to true.
Alternatively Below will display the Thumbnails in multiple Columns and Rows.

Private Sub Browse_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Browse.Click
    Dim OpenFileDialog1 As New OpenFileDialog
    Dim cols As Integer = (PhotoGallery.Width - (PhotoGallery.Width Mod 100)) / 100
    Dim curLeft As Integer = 0
    'added code
    Dim curTop As Integer = 0

    OpenFileDialog1.Filter = "Images (*.BMP;*.JPG;*.GIF,*.PNG,*.TIFF)|*.BMP;*.JPG;*.GIF;*.PNG;*.TIFF|" + "All files (*.*)|*.*"
    OpenFileDialog1.Multiselect = True
    OpenFileDialog1.Title = "Select Photos"

    If OpenFileDialog1.ShowDialog() = DialogResult.OK Then

        For Each file As String In OpenFileDialog1.FileNames
            Debug.Print(file)
            Dim imgFilename As String = IO.Path.GetFileNameWithoutExtension(file)
            Dim img As New System.Drawing.Bitmap(file)
            Dim imgImage As Image = Image.FromFile(file)
            Dim imageControl As New PictureBox()

            imageControl.Height = 100
            imageControl.Width = 100
            Dim myCallback As New Image.GetThumbnailImageAbort(AddressOf ThumbnailCallback)
            Dim myBitmap As New Bitmap(file)
            Dim myThumbnail As Image = myBitmap.GetThumbnailImage(96, 96, myCallback, IntPtr.Zero)
            If myThumbnail IsNot Nothing Then
                imageControl.Image = myThumbnail
            Else
                imageControl.BackgroundImage = myBitmap
                imageControl.BackgroundImageLayout = ImageLayout.Zoom
            End If
            imageControl.BackColor = Color.Black
            imageControl.Left = curLeft * 100
            imageControl.Top = curTop * 100
            curLeft += 1
            If curLeft >= cols Then
                curLeft = 0
                curTop += 1
            End If



            PhotoGallery.Controls.Add(imageControl)
        Next
    End If
End Sub

Public Function ThumbnailCallback() As Boolean
    Return False
End Function