In rec.photo.digital harrogate3 wrote:

JPEG is a lossy form of saving the picture -

The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.

Google on 'explanation of jpg' and it will give you the detail

or read
http://photo.net/jpeg/learn/