Bark is wonderfully organic and varied, it's also easy to photograph,
thus making it a great thing to make materials from :)

Fire up yer camera and take snaps. Now, I'm lucky, lots of great trees near me. Snap away! Watch for sunlight being too bright and forming bright patterns on the bark as you take pictures, avoid that, usually. You want a nice overcast but bright day.

Now, ideally you should make these pictures for textures, square in shape and SEAMLESSLY TILED, it's easy with Paint Shop Pro.

I made a script in Paint Shop Pro9, to make life MUCH easier! Script is roughly:
-Selection (Square) near middle of picture, large as possible, and cutting out any curvature of the tree trunk which distorts the texture.
-Crop to selection.
-Make Seamless tile.
That saves a ton of time!

Lots of other ways to do that with other applications. A great seamless tiling plug in for Photoshop (and paint shop pro), is IMAGE SYNTH

Next, Vue...

make a terrain, edit the material, now, generally since we will use the bark terrain for organic stuff: ground, tree trunks etc, we don't want there to be much Highlights, so turn Highlight global intensity down to about 5% or so. Tweak to taste on specific items later on :)

Load your bark pic into the Colour "Mapped picture" tab.
Now there is a basic material, save it.

Not perfect is it? Useful but lacking...the scale is such it's just too small a pic to apply to a whole large terrain, you may find.

Basic bark...

Note set material mapping to Parametric, and add bump, edit bump function, drive it from the colour, set it to about 0.3

Now, to improve this we can adjust the scale of the image down, since it's tiled it should be fine, but, it's still kind of bland.
A better way to get a good look is to mix several such materials together.

Make several of these bark mats, save them.
Start a new material, set it to be MIXED, combine two bark mats. Note to adjust the blending slider as you wish.
Also, changing mapping and scale ofthe images can all have an effect

This terrain has THREE bark mats combined. The effect is quite nice :)

This is that material applied alttitude sensitive, with a default Vue material as lower terrains, and altitude driven ecosystems on each of them :)