This is a feature only in Curios 4.0.6.0+.

Modders, modpack developers, and users can register their own slot textures to be used for their custom slots by simply placing the textures in the correct location.

Registering a Slot Texture

Create a new resource directory assets/curios/textures/slot and place any .png file in the slot folder.

Note: As of 1.19.3, the directory does not need to be in the curios namespace anymore. This method will work with any namespace as long as the directory is still (namespace)/textures/slot.

For modders, this directory should be created in the mod's resources folder.

For modpack developers and users, this directory should be created in a resource pack or any asset management system such as KubeJS.

The image itself should be 16x16 in size and have a transparent background with only the slot texture drawn.

Example:

Using a Slot Texture

For modders, when using the IMC builder, append .icon(new ResourceLocation("curios:slot/image_name") to specify the icon to use for the slot being built.

For modpack developers and users, specify the icon in the curios-server.toml configuration file:

[curiosSettings](/TheIllusiveC4/Curios/wiki/curiosSettings)
  identifier = "custom"
  icon = "curios:slot/image_name"