There is a lot of examples in the testbed, it's a good place to learn what the engine is capable of producing.
If I am understanding you correctly, you want to use your terrain data (in texture form) as the rendering texture too. This may be difficult, but not impossible, because the physics coordinate system is different from the world coordinate system. If
you're not aware, the physics coordinates are much smaller than xna's world coordinates. So there is a scaling issue that may make it hard to get them to match up...though I've seen posts on the forum about people doing that, so I know you can find something
Doing that can cause difficulty because you can't just take the vertices from your fixtures and set them as-is into vertex buffers. It wasn't covered in the other forum post, but you'll have to convert the fixture vertices to world space before you save
them in a vertex buffer. I use different data to create my fixtures and to render my terrain. My terrain comes from a black and white (or maybe its black and transparent (i can't remember)) texture that is filled in with black representing solid
ground and white being empty. http://img851.imageshack.us/i/mapsn.png/ I then use other textures to render my polygons, so think of a repeating texture of rocks being stamped across my
terrain. That may not be ideal for your game.
I fear that my explanation may send you off into a deep hole if you're not familiar with many of these words or concepts I'm throwing out here. You may want to take things in smaller steps rather than just following my approach. Maybe
you'd come up with something better once you learn more about the physics engine.
If you are still considering what I do, then I'd recommend you implement these things one at a time and then move on because they build off each other.
Forgive me if you already know how to do these things...
1. Render a triangle to the screen http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series1/The_first_triangle.php
2. Understand world space http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series1/World_space.php
3. Understand indices http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series1/Indices.php
4. Texturing polygons http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series3/Textured_triangle.php
5. Use Vertex and Index Buffers http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series1/VertexBuffer_and_IndexBuffer.php and texturing them http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series3/Triangle_strip.php
Doing those tutorials will teach you how to render textured polygons to the screen. As a learning experience you can then work backwards, by converting those vertices to physics space and making fixtures for them. Doing all of this should teach
you how my rendering system works. You can decide at that point if you want to dive into the decomposition and marching squares examples.