The code used by you is not displayed in the posted query, however, following is the code snippet that can be used to add a custom button in the Viewer Toolbar along with the desired image:
Dim prnt_btn As New ToolStripButton
prnt_btn.Image = Image.FromFile("..\..\Print.jpg")
Viewer1.Toolbar.ToolStrip.Items.Insert(2, prnt_btn)