Tom Richardson wrote:... jpegs are 'lossy' - they dump information to make the files smaller. They do that every time that you edit and save them ...
I've read that too. I don't believe it's true - at least not every time you edit and save them, unless you also compress the image. 'Compressing' images for the web (to reduce file size and download time) reduces the number of colours - imperceptibly if done correctly - but this does not happen if an image is saved without further compression. A jpeg image can be saved over and over again without any loss of quality. If you save a jpeg image a thousand times, the file size - an indication of the number of colours - remains the same, so there is no loss.
Of course there are many ways to destroy a jpeg image by editing it!