Does the name of an image downloaded from Facebook (often numbers) represents something or are mere numbers? For example this name – 178696_436095929768187_414916441_o.jpg.
Unless Facebook has changed their naming convention the middle part is the Unique ID of the image while the last letter signalized the size ("s" thumbnail, "n" normal, "o" original).
If that is still the case you should be able to access the same image using the following URL:
Other then that the meaning of the numbers are unknown. The name does no longer contain the profile ID of the Facebook user who uploaded the image. This was changed at the end of 2012.
Ah! Thanks Jan F. As you are saying what last letter represents, can you tell any way to download the original image? Like even if I use "Download" link on a facebook photo it gives me an image whose name ends with letter n. Any way to download every image's original version?
You get the original picture within the Facebook photo viewer when you click on "options > download". If that picture is noted as "n" it means no higher res. version was uploaded.
I'm not much of a FB user so I don't know all the guidelines and stuff but basically depending on where the image was uploaded (profile picture, timeline, image post, photo album) certain limitations in resolution and file size apply. Some are automatically re-scaled on the FB server too.
So for an image in a photo album you should get "original" every now and then. For timeline images or profile it will usually be something smaller.
I suspect only a FaceBook engineer would be able answer that. It is either a random number, or one which has meaning in terms of FaceBook's databases, but as far as you're concerned it's random.