If you do end up deciding to charge money, depending on how niche it ends up being, Aseprite could provide a good model of an (extremely) reasonably priced tool

If you do end up deciding to charge money, depending on how niche it ends up being, Aseprite could provide a good model of an (extremely) reasonably priced tool that fills a specific need: https://www.aseprite.org/ https://www.aseprite.org/ But I also have a ton of respect for the desire to offer it free!