Current wisdom is that there is little point in compressing jpegs for archive or transmission purposes using standard lossless compression formats such as zip, rar or Stuffit. The jpeg data is already so heavily compressed that whilst you can create a zip or stuffit archive of jpeg files quite easily, the resulting compressed archive will be approximately the same size as the original image file, so there isn’t any point.
However this could all change if claims from the makers of Stuffit hold true. Slashdot reports that the next version of their software – Stuffit 9 – will include new technology capable of losslessly compressing existing jpegs by a factor of almost 30%. This is very impressive if true and could have important implications for purposes such as archiving large quantities of pictures and transmitting pictures over very slow links such as satphones.
There’s a detailed third-party test of the new technology here. The manufacturer’s press releases on the technology, which are a bit thin on details, are here and here. However there is a more detailed white paper on the technology here. [PDF file - 89KB]
In the white-paper they propose to create a new file-type based on the technology – SIF or Stuffit Image Format.
The Slashdot article contains interesting discussions on different compression techniques including fractal image compression and the merits of the new Stuffit technology compared to image formats such as the seemingly impressive but little-used Jpeg2000.