I tried changing the PixelFormat but the restriction comes from the BitmapPalette. Here's what I found out up to now:
You could use a Brush that draws an ImageSource or a Visual.
If you want to use the ImageSource, you have certain possibilities:
-You could draw a Drawing which offers the possibility to draw an ImageSource (among others which I don't think about that they will be useful)
-You could draw a Direct3D image which does not seem to be what I want
-You could draw a BitmapSource (which has certain subtypes as shown in your link). But each of them has a BitmapPalette which is restricted to 256 colors
If you want to use a Visual, you also have different options:
-A Viewport3DVisual does not seem to be the right thing
-You can draw a Drawing which results in drawing an ImageSource
-You can draw a UIElement. Maybe I could use a Canvas on which I have drawn an Image before, but I wonder whether there is no way to use an Image or Bitmap (not BitmapSource) directly?