For realistic looking eyebrows a high resolution photo source can't be beat. The next best thing are custom Photoshop brushes that simulate hair.
The image below is the result of one method I use for isolating hair from skin in a photograph. This works well on high resolution skin templates (2048x204

where individual hair follicles can be isolated. What is shown below is just a sample from a modeling photograph, reduced for the forums.
Using Photoshop:
1. Copy the original image to a new layer
2. Use the Rubber Stamp and Healing Brush to sample out the original eyebow hair.
3. Perform a difference calculation by setting the layer mode of the sampled layer to Difference.
4. Select All, Copy Merged, and paste the copied image on a third layer. This will look mostly black with some colored detail in the spot where the eyebrow was.
5. Clean this third image up a bit with a Level adjust, Hue Saturation adjust, and maybe a few swipes with the eraser and a custom brush tool.
6. Copy this image over to a new channel.
7. Create a new fourth layer using the new alpha channel as a selection mask and fill it with pure white.
8. Set up some layering effects on this 4th layer. Good ones to add are Color Overlay, Bevel & Emboss, and Inner Shadow.
Playing around with layering effects can get nice color variations.
Another more complex method of isolating hair is with a 3rd party masking utility. I still use the old Extensis Mask Pro sometimes, but PSCS3 has some very good masking tools by it's self. This method is more time consuming, but is better for hair that subtly blends into the skin.
Some other things you can do with the alpha channel is cut away the sampled image where the eyebows are and produce an opaque skin with transparent eyebrows. The transparent areas will be reactive to any slider changes you make inside SL while wearing the skin. This is a great way to introduce slider adjustable eyebrows into SL on a much more realistic looking eyebrow. It'still not as realistic as working with the original photo, but much, much better than the default eyebrows.