There are two types of Custom NPCs - a NPC which its type is Custom (You hire at Academies) and a Custom NPC which you submit in the NPC request tool and is created at your location.
To get a custom *image* on any of them, just click on your CPs, and on to the "Exchange CPs" page. From there, scroll down to the Custom Image section and read the rules about custom images (important). Now you can select which type of entity you want to submit a custom image to (Room, Ship, NPC etc.), and put its ID number, and a link to a small image (100x100px) and a large image (400x300px to 600x600px).