|I came across this link today about guetzli, a JPEG encoder from Google. Looks promising I think and I wondered what it would do with DPC sized photos. I gave it a try (on Linux) and here are some results
1. 722Kb, Standard JPEG compression, 100%
2. 671Kb, compressed nr.1 with guetzli -quality 100
3. 284Kb, Standard JPEG compression, 84%
4. 283Kb, compressed nr.1 with guetzli -quality 95 (the default)
5. 95Kb, compressed nr.1 with guetzli -quality 84 (lowest value without changing the source code)
Personally I see not much difference (except for the size) without zooming in, but perhaps you do.
Btw. It can't handle photos straight from the camera, at least not the ones I've tried, which is weird since the docs says: "Note that Guetzli is designed to work on high quality images (e.g. that haven't been already compressed with other JPEG encoders)"