Hi. Hope you don’t mind, I moved this topic over to the Zero Brush forum. If you mean the UV/Image Editor (learn more about Blender editor types in this vid: https://youtu.be/QAUm_E7ZiQM). Yes you can paint directly onto the texture images. ZB makes this easier by adding the option “Edit UVs” in the Zero Brush options menu (access by pressing Q or press the “Options” menu in the ZB layer menu and you can also access this menu by pressing the blank button in the ZB icon bar).
After pressing Edit UVs, press T to bring up the toolshelf (while in the UV/Image Editor) and press the Sensei tab on the toolshelf tabs on the left side of the screen to display the ZB menu for editing texture images directly. Additionally, make sure down in the bottom header bar of the UV/Image editor next to where it says “Image” to click the button with the image icon and select the layer you want to paint on. When you’re done you can press the big “Go Back” button at the top of the Zero Brush menu to go back to the 3D view. There is also an “EDIT UVS” button to make editing the UV map more accessible. Apologies if I’ve misunderstood what you were wanting to do. If so let me know. Thanks.