habitica/common/img/project_files/npcs

When adding a new NPC, do the usual. When temporarily replacing an NPC with an event (halloween, thanksgiving, etc) - you replace the image files in img/sprites/spritesmith/npcs, and keep all designs in this folder so we can later replace back after the event is over. This is the most size-optimal approach (vs having all npcs in the spritesheet & referencing them differently in code)