With ZBrush open, hit CMD-Shift-3. You’ll find a PNG file of your screen(s) on the desktop. Open it in Photoshop. Go to Image -> Image Size and type in 900 or 1000 for width, make sure pixels is selected and “resample image” is checked. The go to Fil -> Save for Web and Devices (or just Save for Web) and save out a JPG.
That will give you a file that’s plenty readable without the hassle of using Grab.