for (int x = 0; x < this.imageListToolbar3small.Images.Count; ++x)
        {
            Image temp = this.imageListToolbar.Images[x];
            temp.Save(this.imageListToolbar.Images.Keys[x] + ".png");
        }

foreach (ToolBarButton b in toolBar.Buttons)
{
  //can be negative, for separators, because separators don't have images
  if (b.ImageIndex >= 0)
  {
    Image i = toolBar.ImageList.Images[b.ImageIndex];
    i.Save(b.ImageIndex + ".png");
  }
}

Dim cntrl = New TheClassWithThePrivateImageList
    Dim pi As Reflection.PropertyInfo, iml As System.Windows.Forms.ImageList, propName = "ThePropertyName"
    pi = cntrl.GetType.GetProperty(propName, Reflection.BindingFlags.NonPublic Or Reflection.BindingFlags.Instance)
    iml = CType(pi.GetValue(cntrl), System.Windows.Forms.ImageList)
    For Each key In iml.Images.Keys
        Dim image As Drawing.Image = iml.Images.Item(key)
        image.Save($"{propName}_{key}")
    Next