You have to create a tileset first (.tsx), load the png and then do what I explained in my previous comment by selecting the tile from the tileset, open the animation editor and then add all frames. It's also helpful to open all my .tsx and example maps to see how things are done. To learn the software, you should also check their documentation (https://doc.mapeditor.org/en/stable/) and forums (https://discourse.mapeditor.org/). The software is easy once you get the workflow.