If you're using Poser 6, then the node you want is the color math node.  Set it to the bias function to adjust brightness or the gain function to adjust contrast.  Run your texture map into value 1, then use value 2 to make the adjustment.  For bias/gain 50% grey in value 2 is no change, less than 50% is darker/less contrast, more than 50% is brighter/more contrast.

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