There are a few different ways you can add a reference image to zBrush. But the one that I personally like the best is by using the spotlight feature. It allows you to add any number of images over your work area and conveniently hide or reveal it whenever you need to view your reference. Check out this short 2 min zBrush tutorial and try to use this method in your own work pipeline.

Please share your own experience and feedback below! Happy sculpting!!! =)