Also, There is a problem that some beginners editors have in KS...
Dont put your custom tilesets in Data folder
This will make that the people wont see your tilesets if you relase it, and also, probably you cant play very well some levels that have that tileset.
To fix it, put your tilesets in Knytt stories/Worlds/*the name of your level here*/TilesetsYou have to create that last folder, or you can use the Data Manager as well.