What I did was make a 4K image for my watermark and save it,
Then, whenever I want to use my watermark, I import my source image as a layer in the painting, drawing, etc. that I’m working on.
Then scale that layer to a proper size and make it transparent.
The only requirement is that you need an image editor with layer support, like Gimp, PhotoShop, etc.
Mostly the same as what everyone else said…
Also, if you’re interested in protecting your work, adding copyright info to the metadata of your photos would be a good idea.