That seems to be true only of certain URLs. For instance, if it's all one long, encoded string follow by _a or _s then changing a or s to n produces the full-size image. However some thumbnails aren't formatted in this way and as far as I know can't be (easily) resized to full. Someone please correct me if I'm wrong.
Edit: it appears this same char is available in both URI formats I was referring to, so yes, full-size images are exposed.
To get the full, full size image try changing from _n to _o. I haven't checked it on the links exposed by discussed method (I'm going to work in a minute), but it's how "Download" link for photos works on normal Facebook photo browser - if you upload a high-resolution picture to Facebook, it's available with _o in the name.
[quote]
right click
inspect the image
copy url in another window
change the 'a' to 'n' (the last one right before jpg)
=fullsize original image
[/quote]
http://imgur.com/a/PrLrB (ps: what a nice photo of them on halloween. very generous too I can see!)