Alpha channel generation via JS code

Hi guys. I've looked for an answer elsewhere with no joy, so apologies if someone else had asked this before. I want to make a programme in unity, where the user can add their own picture files, which can be used as materials. Whilst I can use photoshop to convert a pictire to greyscale, invert it, and use that as an alpha channel, I want this to be able to be used by anyone who can knock up something in Paint, or even take a picture of a drawing. Providing I can work a way to fetch the files at runtime (different question folks!), it means I've got to find a way to automatically generate the alpha channel, so it's easy, fast and user friendly. In short, can if be done? I know how to access the material and assign it in JS, but I'm at a loss for how to achieve what I'm after. Thanks in advance. Ian.