This video explains how to use the ZBrush reflection shader (a custom material) to produce interesting reflections on eyes.
Creating your own reflection images is pretty easy actually. You can use literally any image you want, the trick consists of distorting the image to simulate the curvature of the eye.
For best results try to use an image that has lots of ‘dark’ areas, I’d recommend at least 80% dark areas on the image like this example. (you can grab it here)
Try to also find an image that the dark areas are desaturated. I’ll just tweak the image a bit in Photoshop using the Hue/Saturation image adjustment and paste it into a square image.
Now, in Photoshop, go to the Filter menu, then Distort and choose Polar coordinate. A new window will pop up where you can choose ‘Rectangular to polar’ and click ok… that’s it!
You can now save the image and use it in ZBrush with the Reflection Shader just like I showed you in the video. Here is the result of this process with the image above in case you want to use it (I also rotated the document 180 degrees so that the white area is the reflection shown at the top of the eye):