Can you show us what specular shaders are doing. Is the diffuse map plugged into the specular, at all? If so, this is incorrect. Specular highlights on skin are primarily a surface effect, not interacting with the melanin particles beneath, so light and dark skin should have more or less identical specular settings and maps.

Can you show what lights you have in the scene?

There is another option within Poser for adjusting the intensity of an image directly within Poser, before you commit to editing the image elsewhere. On the image_map node, there is a Texture Strength setting, normally set to 1.0. If you reduce this, dark areas become lighter. At 0.0, the image map will be white. Don't trust the previews in the material room, they do not always represent what the texture strength will do in a render. Be aware that this is a mostly linear transform, so you can't do much adjustment without the texture becoming washed out, as the light areas get brighter as well, and less saturated. A Color_Math node set to bias might be a more flexible adjustment if you don't get what you want with Texture Strength.

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