When the filament is cold, you can see it because of light that comes from it after having been reflected off of it, rather than having been emitted from it originally; or, if it's dark on a light background, because of light that comes from other parts of the lightbulb after being transmitted through it from behind. In all of these cases, what you see in the photo is light that came from the lightbulb, and all of them are equally legitimately "pictures of the lightbulb", although they might display more or less detail.
