I submit environmental reports to a local authority. It says: "TIFF files are required for archival purposes as they are uncompressed which means they retain image detail and are stable (ie, do not degrade through successive opening and closing and saving)" unlike JPegs. David Lynn
JPeg files are "lossy" -- they use a compression system that loses some detail -- but opening and closing images does not cause any degradation. However, loading a .jpg image into a paint program and saving it can cause degradation, even if you don't change it. There is no reason to do this, of course, but paint programs typically have a quality setting somewhere, so you will be re-saving them at "95% quality" or whatever. The rule is therefore not to mess about with original images, only with copies.
TIFF (Tagged Image File Format) is a very old but very flexible format and it provides the option for lossless LZW compression, for example.
