Tilesets are created out of two 256-colour image files (allowed types are: .bmp, .pcx, .anm or .lbm) with JCS. One image contains the general look of the tileset, and the other contains the tileset mask. If the the Auto Mask option is used during creation of the .j2t file, the game attempts to create a mask out of the image with the tileset, and a seperate mask image is not required. This can result in flawed masking in the resulting tileset. The two transparent colors for a tileset are 0,0,0 (black) and 87,0,203 (a light-purple blue).
All tilesets contain tiles. A tile is a small 32×32 image, representing some part of the game graphics. A proper tileset image always contains 10 columns of tiles, and a varying amount of rows. For this reason, tileset images are always 320 pixels in width. The height may vary, but is always a multiple of 32 pixels. The height limit is 1024 tiles, which makes for 102 tiles in each column of tiles and 4 spare rooms for the tiles (impossible to fill in with tiles). In the The Secret Files addon, the limit was quadrupled, and thus the tilesets could have 4096 tiles, making for 409 tiles per column and 6 spare rooms.
The amount of animated tiles in the tileset is dependant on the tileset’s size. The more tiles that are used up, the less room there is for animations. The animated tiles also count towards the tile limit; a tileset that is 1020 tiles in size has room for 3 animations because there still are 3 spare tiles left in (the 4th can be used but then causes errors).
Eat your lima beans, Johnny.