Transparency color in image control is black

Question

I develop app in WPF. I have a bmp images I merge together and final image bind as a source to image control. The bmp images are same size. The image control I need as a part of treeview.

Binding of images, treeviewitem.

<StackPanel Orientation="Horizontal">
     <Image Margin="0,0,4,0" Source="{Binding ImgHeader}" Width="16" Height="16" />
     <TextBlock Text="{Binding Header}"></TextBlock>
</StackPanel>

This is a code I merge bmp together with another bmp.

Bitmap oSource = LoadImage(imageName); - image from resource
oSource.MakeTransparent(System.Drawing.Color.White);
using (Graphics oGraphics = Graphics.FromImage(dest)) 
{
     oGraphics.PageUnit = GraphicsUnit.Pixel;
     oGraphics.DrawImage(oSource, new Rectangle(0, 0, dest.Width, dest.Height));
}

And conversion to BitmapImage to bind to image control.

using (MemoryStream memory = new MemoryStream())
{
     bitmap.Save(memory, System.Drawing.Imaging.ImageFormat.Bmp);
     memory.Position = 0;
     BitmapImage bitmapimage = new BitmapImage();
     bitmapimage.BeginInit();
     bitmapimage.StreamSource = memory;
     bitmapimage.CacheOption = BitmapCacheOption.OnLoad;
     bitmapimage.EndInit();
     return bitmapimage;
}

But when the final image has some pieces white color, the result of these pieces in app in treeview is black color. This is what I see in the treeview.

White color has to be on top of images on the picture below. This is what I want.

I need white color. Where is the problem?