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As it's WPF, you can place an image above the media element, and if it is a PNG with transparency, your image will appear above the video. You set Canvas.ZIndex on your controls to set their order, which is far from intuitive IMO.
*Edit by Mark S*
I usually use a Grid to overlay an element on another element as Christian described, something like this:
<Grid>
<MediaElement />
<Image />
</Grid>