Yes, I've got the reason why..
Please, try this code:
RadForm rf =
rf.BackgroundImage = Image..from..
rf.BackgroundImageLayout = ImageLayout.Tile;
RadSplitContainer rp =
//RadPanel rp = new RadPanel();
rp.Dock = DockStyle.Fill;
rp.BackColor = System.Drawing.Color.Transparent;
rf.StartPosition = FormStartPosition.CenterScreen;
Just try to un-comment the line and you'll see that in case with SplitContainer it doesn't work.