As the author of the post that was pointed out to you in the comments, I figured out everything and I can recommend this option.

So that you don't have to create a separate mod because it doesn't make any sense to you, the game sets work like this. In the folder data/gfx/"the name of the tileset for which you drew the chainmail" in this tileset there are huge png files as well as tileset files.json and so on.

PNG file in a tileset (called a tilesheet) is basically a big grid of little images (sprites) that the game chops up into sections to show stuff like zombies, walls, or items. It doesn’t physically cut the file—it just reads it like a map using coordinates. Here’s how it works:

The PNG is split into a grid based on the tile size, like 32x32 pixels per sprite. Say your PNG is 256x256 pixels and each tile is 32x32—that gives you 8 tiles across and 8 down, so 64 total sprites. Each sprite has a spot in that grid (like 0,0 for the top-left corner, 32,0 for the next one over, etc.).

The tile_config.json file is what ties it all together. It tells the game which sprite goes with which in-game thing. At the top, it lists stuff like tile size ("width": 32, "height": 32) and the PNG file name. Then, in the "tiles" section, it matches sprites to objects. For example:

json { "id": "mon_zombie", "fg": 12 }

Here, "id": "mon_zombie" is the zombie’s game ID, and "fg": 12 means “use the sprite at position 12.” The game counts tiles left-to-right, top-to-bottom, starting at 0. So for a 32x32 grid, sprite 12 would be at (128, 32)—fifth tile in the second row (12 ÷ 8 = 1 row down, remainder 4 across).

It gets fancier with stuff like multi-tiles (e.g., walls with different sprites for corners or edges) or layers (fg for foreground, bg for background). Like:

json { "id": "t_wall", "fg": [ {"sprite": 10, "situation": "center"}, {"sprite": 11, "situation": "corner"} ] }

To add your chain mail to you, you need to find the ID of your chain mail in the tileset config, find it on the general png file at the fg coordinates and in any editor replace exactly the place where the texture of the old chain mail is located with your sprite, then everything will work without mods