Digital Image Basics for non-Photographers • Blaise Tobia
Each re-save requires a whole new analysis of the image by the JPEG compression routine,
so resolution loss is increased and artifacts become enhanced.
The following three images show deteriorating quality as the same file is re-saved as a JPEG multiple times:
re-saved as Q7 - 3 times
re-saved as Q7 - 9 times
What this means is that images should be saved as JPEG files only as the last step.
They should be maintained as non-compressed files (PSD, TIFF, PNG) as long as they may need to be be
re-sized, cropped, color adjusted, sharpened or otherwise modified.
Only when the image is ready for submission, should it be saved as a JPEG of the specified quality.