Open a new canvas, white 800x600, and draw a black rectangle 600x400. Right click and Convert to Image. Go to the Easy Palette and choose Photo Frame/Edge Preset, I have used here the edge 26, and double click on it and deselect. Now you can decorate the mask, dingbats, flowers, leaves or whatever. Make everything 2D and black. When you have it to your satisfaction merge all, convert to grayscale and save as jpg.

Now it's the fun part, open your mask in PI. Open your EP and create a new folder, in my EP the folder is "My Masks". Click on Selection/ Import selection. Then Selection/Invert, and last Selection/ Copy selection to Object Library. Open your photo in PI and double-click on the mask in EP. Right click on the mask and Convert to Object and drag the mask to a new canvas, deselect and fill the background with a nice complimentary colour.

That was all. Hope you enjoyed the tutorial.