Here’s how to create a perfectly fitting mask for a frame that has none. The directions are for Photoshop, but most image editors work very much alike.
1. Open the frame in Photoshop.
2. With the Magic Wand tool, select inside the transparent area of the frame.
3. Go to Select > Modify > Expand and type in a number somewhere between 5 and 15. (You want the selected area to expand and cover a bit of the frame’s inside edge.)
4. Create a new, blank layer above the frame layer.
5. With the Fill tool, fill the selection in the new layer with white.
6. Go to Select > Invert so everything BUT the center of the frame area you’ve filled with white is selected.
7. Fill what’s now selected with black.
8. Go to Layer > Flatten.
9. Save AS under a new name. (I usually add “MASK” in caps to the end of original name of the frame so mask and frame stay together in a file listing and it’s obvious which is which.)
You now have a grayscale mask that’s a perfect fit to the frame.
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