In this instructable you will learn how to make realistic wood in Photoshop or GIMP. The steps might not be exactly the same for GIMP as they are for Photoshop. Photoshop (for Mac OS X) is used in this instructable.
Step 1: Create a New Image
I made a 640 x 480 transparent image but it really doesn't matter what you do.
Step 2: Fill With Color
You will want to fill your blank image with a brownish orange color. The values of my color is R 205 G 150 B 0. You can use that or what ever you feel like using. Then use the paint bucket tool to fill the image with color.
Step 3: Add Noise
Add some noise. Go to Filter > Noise > Add Noise. Use the following settings. Amount 7.35%, Distribution Gaussian, Check the Monochromatic box.
Step 4: Motion Blur
Add some Motion Blur. Go to Filter > Blur > Motion Blur. Use the following settings. Angle 90 degrees, Distance 25 pixels.
Step 5: Make and Desaturate a Duplicate Layer
Make a duplicate layer. Go to Layer > Duplicate Layer. Then go to Image > Adjustments > Desaturate. Them go to the Layers palate and Change the duplicate layers Opacity to around 40% and change the Fill style to Soft Light.
Step 6: Poster Edges
Add Poster Edges. Go to Filter > Artistic > Poster Edges. I used the default settings which are Edge Thickness 2, Edge Intensity 1, Posterization 2.
Step 7: Liquify
To Add some knots liquify the image. Go to Filter > Liquify. Use the Swirl Tool to make some knots and such. You can try messing with the other tools in there too.
Step 8: Finished Product
Now do what ever you want with your wood.