itch.io

For the most part, there are no “strict” rules when publishing your work, as long as you own the rights to it. The watermark is just a suggestion, so people can’t just steal your asset without paying (assuming it’s a paid asset).

The watermark can be anything that sits on top of the asset in such a way that it can’t be removed easily.

If you plan just to show-off your asset without selling it (so for free), then I’d say just go for it. You wouldn’t need a watermark in this case. Just make a simple page and keep adding on it over time.