The curse of the JPEG - for fine art reproduction.
JPEGs are a neat way to make a file smaller. The process was originally invented for uploading pictures to the web as the normal high-resolution, wide colour range of image files was unnecessary for viewing on the average PC screen, the file size also had to be as small as possible for use with the internet.
JPEG works for standard printing, as the CMYK process had a limited colour range, and high speed/low cost printing is a compromise on quality itself.
Where you need the finest quality and most faithful reproduction, and file size is not an issue, Saving an image as a JPEG represents a big compromise.
So how does JPEG make a file Smaller? Well, instead of each colour/tone having its own reference, JPEG ‘averages’ colours; it groups similar tones together and then assigns a group reference. This makes the data, or file size, smaller. The higher the compression, the bigger the group, the less colour detail. Of course JPEG has different levels of compression, the process that makes files smaller but, as most people use it to make the files as small as possible, they ‘squeeze’ all the colour detail out in the process.
And its this colour detail that differentiates a giclée print from standard printing. The number of tones and colour range (gamut) are much higher with giclée printing So, if you’re sending a file for giclée printing save it as an un-compressed file, a TIFF or PSD. That way you’ll get the most faithful print at the highest quality.
Sadly, there no way to ‘recover an image saved’ as a JPEG once its been compressed. The data is lost forever. Re-saving a JPEG as a TIFF or PSD will not improve the quality and the detail will be lost forever.