Well, better use a raster software that supports layers. GIMP is what I use personally, but I’m sure people here use other stuff like Paint.Net and even Photoshop. ;)
Inkscape would work, but since it’s made to deal with vectors instead of raster images, it’s less than optimal.
I used to do it in PMs constantly. Then the forum software updated and notifications became a thing. Now it’s more public and there’s an aspect of public shaming to it. Which, well, isn’t necessarily nice of me, but does seem to work better than the old PMing days.
Again, my apologies to anyone I annoy the crap out of. I just like to keep the game going.