Esme_Belles said:
How the he'll does that actually work? I remember there was a handful of posts a year or two ago that had entire site rips inside them, most of it DNP content. But how does that even work... and how do you recognize when that occurs? I remember that I was totally confused, I'd DLed a post just to see what everyone was yelling about and ended up deleting it from my HD cuz I couldn't figure it out...

A lot of archive formats / archivers (eg.zip, rar, 7z) are designed so that the start of the archive data doesn't need to be the start of the file.
So you basically just concatenate the jpg file with the zip file;
on Linux that's as simple as cat myimage.jpg myarchive.zip > image_with_archive.jpg.

Because that tolerance is built in to archivers, you just unpack it like a normal zip
(unzip image_with_archive.jpg)

In terms of GUI interaction, I think it's just a matter of asking for the file to be opened in your archiver rather than your image viewer.

Image viewers also generally have the converse necessary property -- they ignore 'junk data' beyond what the image headers tell them to read.
So that side of it also "just works" (in general - jpg, png, tiff etc. GIFs don't seem to be compatible with the concatenation strategy, although if you are fine with the image side of the content being broken, you can still use this method). The image viewer sees the file as an image, the archiver sees it as an archive.