It is recommended that all tiles use transparent channels to obey tile index sorting.
The standard save format of the library is a row of 16 tiles, if there is a variant, placed directly below the corresponding position.
Ideally you should provide sprites with transparency channels.
For tiles in non-standard cases, conversion functions are provided.
Yes, this resource format will have a lot of useless transparent pixels.
But this is just a lossless resource format for editor, it will compile into a compact format in game.
And thanks to the png compression algorithm, it doesn't take up too much extra disk space