Seems the taskbar transparency is based on the background image, regardless of what may actually be behind it. Loading any full screen app in windowed mode, then using the windows key to bring up the start menu will show the background image through the transparent taskbar, not the app behind it. Additionally if you turn on hide the task bar, whenever the task bar pops back out it still shows the background image, not the apps it's covering up. The window transparencies don't have this problem, as they properly show and update any app behind them (even a D3D game)
