Document (Layers): The document IS an image, so you can just import your reference image on the bottom layer and edit the model on a second layer. You can also draw multiple images onto the document by using built-in tools like Simple Brush or Image Planes (see below). This method won't get changed by your view of the…
cryrid already pointed out all the options ZBrush gives you. What you could also try, and what I am using on a regular basis, is using a reference board tool like: Kuadro or PureRef (which I prefer) These let you overlay images over your other applications, not only ZBrush. In PureRef you can also lower the opacity of the…
If not, I guess the devs presume that all of their users are using 2 monitors? Btw: I'm not talking about setting up a textured plane in the document, that's just time consuming madness. That's the technique suggested on Pixologic's site but it just seems insanely complicated -like 11 steps (and gets affected by your…