Solved Custom masks

Derek, great seeing your renewed activity and Q&As!

After seeing Derek's fine tutorial on making for high contrast clips, I just watched an excellent tutorial on using graduated masks (). It reminded me about creating custom masks. In MS, there is no tool for painting a custom mask. However, this can be done in even a fairly inexpensive or free image-editing program (discontinued Serif Picture Pus 8 and has show-me tutoials, for $25, and GIMP, for instance, is free but difficult for the inexperienced). Does it work to create, for instance, a 1920x1080 jpg file to use as a generated media file in HD, or is there a pixel difference? One can save a frame shot in MS and place it as a layer in the image editor, then customize a gradient or paint a mask of any shape. Tracking a moving video underneath, of course, might require extensive envelope work, but for relative still requirements, this could be the ticket.
Comment or advice?

I'll update my own post. I got a chance to experiment last night. I found that I needed to create the custom file as 1920x1080 at 96-pixel resolution in order for it to match the captured video frame which I put on a layer underneath the custom mask so that I could paint the mask sufficiently accurately. I saved the mask (opaque to transparent) as a png, imported it back into MS, and it behaved just like the Sony mask, as covered in the above-linked video.