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Modding 101 Chapter 6: Creation of a .tmod page
You have two options:
1. is to make a preview image, formatted with the mentioned size, which you put in the folder ../ui/override/{filename_of_preview}
2. use the _ui blueprint, which, in case of a collection mod, you already made anyway. This should be in ../blueprints/override/{name_of_blueprint.blueprint}