OK, now that I understand a little better what is going on, I see that my initial answer won't directly work for you. However, I also see that you have a bit of a problem.
Just the general nature of the way OnRender works means that what you draw is always going to end up behind the images and whatnot that you add to the window.
Add to that the fact that you're putting all this drawing code for a specific feature (the progress clock) into the window itself, and this solution feels a little off.
You might want to explore some alternatives.
A simple one would be to create a UserControl to draw the Clock. That UserControl could have a DependencyProperty for the % that it should be filled. You could use your (roughly) same OnRender code in the UserControl or you could do it some other fancy ways (I'm sure there's some way to do it in all XAML, though I don't know it off the top of my head). Then you just put that clock into the window like all your other images/controls.
You could also do it creating a CustomControl, though that takes a little bit more knowledge about WPF and Resources and whatnot to understand how it works. Since you're new to WPF, that might be a bit much right now.