TreeView Run-Time ImagesList

//Set the StateImageList of a TreeView to a new ImageList, at run-time
// imageList1 is used at design-time to setup the TreeView
// Thes ImageList is supposed to have its ColorDepth set to 32 bit ARGB
treeView1.StateImageList = TreeViewImageListRunTime(treeView1, imageList1);

// Dispose if the original ImageList is no longer needed
imageList1.Dispose();

// [...]

        private ImageList TreeViewImageListRunTime(TreeView tv, ImageList imList)
        {
            ImageList cloned = imList.Container != null
                ? new ImageList(imList.Container) : new ImageList();

            // ColorDepth set to 8 bit indexed (could also be 32 bit)
            cloned.ColorDepth = ColorDepth.Depth8Bit;
            cloned.ImageSize = imList.ImageSize;
            cloned.TransparentColor = tv.BackColor;

            for (int i = 0; i < imList.Images.Count; i++) {
                var bmp = new Bitmap(imList.ImageSize.Width, imList.ImageSize.Height);
                using (var g = Graphics.FromImage(bmp)) {
                    g.FillRectangle(new SolidBrush(cloned.TransparentColor), 0, 0, bmp.Width, bmp.Height);

                    g.DrawImage(imList.Images[i], Point.Empty);
                }
                cloned.Images.Add(bmp);
            }
            return cloned;
        }