Slot Icon Textures
A tutorial on how to add custom slot textures to slot types.
Overview
Registering a new custom slot texture can be done through a resource pack. If you are unfamiliar with resource packs, it is recommended to read through the wiki page in order to understand the concept and structure before proceeding to the rest of this page.
Registering a Slot Texture
Create a new resource directory assets/(namespace)/textures/slot and place any .png file in the slot folder.
The image itself should be 16x16 in size and have a transparent background with only the slot texture drawn.
Example: 
The (namespace) field should be replaced by the namespace chosen for this portion of the resource pack. If this is a
part of a mod, the namespace is usually the mod ID. If this is part of a user-defined resource pack, then the namespace
can be anything that is lowercased with no special characters.
Using a Slot Texture
When entering the fields of the .json slot registration files as described in Register a Slot Type,
the "icon" field should be set to the path of the slot that was added to the resource directory.
For example, a empty_test_slot.png file placed in assets/test/textures/slot that wants to be added to the charm
slot should have a charm.json slot registration file in a datapack with the following contents:
{
"icon": "test:slot/empty_test_slot"
}
Note that the path in "icon" is not the entire path of the directory. It omits the texture part and also omits the
.png extension. This is due to how Curios and Minecraft parses the icon, so make sure the path is formatted correctly
when adding this field.