Interestingly, a few years ago I discovered that you could also change the "t"/"s"/"n" (tiny/small/normal) to a "b" (big) and access the full-resolution image which was uploaded.

They seem to have discontinued the "b" prefix shortly after I found it (perhaps to save space?). It was a bit of a privacy concern that they retained the full-resolution image, considering that most users probably do not crop their images before uploading them.