I haven't actually built WPF apps on 4.0 that deal with media but I did in the 3.5 bits. Actually, I worked in this space for a year and a half, and unfortunately, you're not going to be able to do it without creating your own graph (and a bunch of back-and-forth with DirectShow) or using a third party library to get you started.
] that looks promising for what you're trying to do (and is written by an active blogger). The project has current posts on CodePlex.
This is just a technicality, but a pure WPF solution is not possible. WPF is really more about the display, not file I/O and image processing. You can use it to render the current state of you innards while chugging through your frames, but stand-alone WPF can't do this one.
Good luck mate.