> I don't get why it'd be set up like this

The add-ons switches to using the CBZ (Comic book Archive) format to be able to snapshot even very large websites completely. (as show in the 3rd preview image on the store page) Other add-ons are generally not able to do this because those are using the canvas API which imposes hard image buffer limits that prevents them from being able to allocate enough memory. AFAIK, even the Firefox built-in screenshot tool is limited by this restriction.
Also the reason you might be able to rename the .cbz file to a .jpg and it still opening as you'd expect might just be because the image viewer you are using supports the CBZ format, but thats just my guess. What i know is if it has the .cbz extension it'd definitely not a .jpg. 😊

I would be nice if you would consider amending your review at least a little after reading this explaination and if you have any more questions or improvement suggestions, please feel free to open issues at the support site.