Joe
My background is photography and 2d digital graphics, I have been trying to solve this, but so far the problem seems to be in the cad/cam software.
I have found that for me the issue is apparently in Aspire. It uses 65,000 steps of gray to describe the height field, but when you import a texture it will only see 0 - 255. That is why you get only a shallow depth change. There also seems to be an awful lot of "noise" regardless of how big or clean the bitmap file is.
I took the liberty of altering your plank file to enhance the grain and importing it. The attached images show the 3d preview and the expected output when cut with a .125 ball nose at 9% step over. It isn't bad, but it isn't right either.
If anyone has any ideas about how to get the cad programs to accept files in a different way i would love to test it. I can create a B&W file with any characteristics needed including 16 bit depth, just can't get it into the CAD program.
Bob