"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."

This also works in P5.
There are lots of ways to alter the way an image map works in P5/6, (eg, to darken an image, just plug it in as usual, then reduce the value to less than 1.0, or to lighten it, increase the value above 1.0) - or change the diffuse node's colour (which can also add a "colour cast" to the texture) ....

Alas, I' m in no condition to add extra advice - maybe tomorrow... -- must go to bed now...

Cheers,
Diolma