Masking a whole image in texture mode I'd like to know how I can mask (if possible) my whole image in texture paint mode, so at the end, I don't get unnecessary paint and colors in my diffuse texture.
I wanted to put the following image as a mask to my diffuse texture, but seems like I can't find the right place to do it
Black and white images can be used as a Mask in the texture paint mode, setting them up in the Slots panel. 
The cool thing is that you can use that image you have there as a mask, and invert it to paint outside that area as well.
video on mask use with mask tool and stencil mask