Hello,
For those who don't know the stamps feature in Substance Painters is amazing!
All you need to do is drag and drop your 1:1 square image to the shelf as an Alpha (I used to do like that and it works).
After that, I can drag and drop again to my mesh in any position and select a few options like use it as a mask, base color, etc.
But the thing is, if you drag and drop to UV Viewport, you are limited to it.
Same thing with 3D Viewport.
But I often want to jump from one to another and can't. Because instead of warping in 3D viewport I can follow the UV
So please, both viewports have different options and both would be much more powerful together.
Peace!