Tell me a little more about the screen mask. Are you hoping to add a mask to your monitor? So that when you're recording or capturing an image you cannot see anything that's underneath it? I added this Dragon to my desktop. It's just a PNG image that sits on top of everything. I could've used an image that was square-shaped, or round. It could be a color or any image. I used the Dragon for the heck of it.
Are you trying to create a mask in the editor? Or what exactly did you have in mind?