You don't really have to "proof" that the car is yours... I did believe Atom and Ingo without phoning the Ludolfs or asking for identity. To apply a car here, there should be a usable photo, nevertheless. If there is at least a recognizable photo and the correct data in the movie page and the car is not modified anyhow (like atoms's ford for example; missing front grille, whitewall tires and I think lowered suspension) I might find enough pictures of the same type anyhow. (You see the comic is in black & white, anyway.) Adding cars to IMCDB isn't that difficult and explained on the imcDb main page.