It could be either. For example, I believe zpaq or 7z or one of the super-cutting edge compression tools actually does compress JPGs losslessly and provide you

It could be either. For example, I believe zpaq or 7z or one of the super-cutting edge compression tools actually does compress JPGs losslessly and provide you bit-for-bit the same binary file you put in (but saving 10-20% inside the archive).