If you're looking at the 'simple' setup in the material room, you'd want 'transparency'. You can click on the square to load the image you want to use for transparency. In the advanced tab, it's the line that says 'transparency'. You want to create an 'image_map' node to attach there (black background, black for what should be invisible, white for what you want seen, shades of grey for partly transparent things). Also connect this image_map to 'transparency_edge', which should be below 'transparency'. Set this to 0.0. You will also see 'transparency_falloff', and by default this is set at 0.6 -- set this also to 0.0 for best results with eyebrows.

-D
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It's all fun and games until someone loses an eye texture.