At some point in the creative process someone, may it be a client, peer, or boss, will ask you to make a change to your work, And if that work is an image, you may be tempted to make that change by opening up the jpeg and making the edit ’cause it’s faster than simply opening the original file and making the change and then re-saving the jpeg (or whatever your final format is). And no matter how much time it’ll save you, here’s the lesson: don’t. Don’t ever work from a jpeg or any other compressed format. After the jump you can checkout a video that shows a jpeg that’s been opened resaved 600 times and you’ll see exactly what each save does to the image’s quality.
So there you have it. Re-saving a jpeg really is bad practice and now you know why.
If you’re really geeky, you can check out the actionscript source and do it yourself (it’ll be faster than doing it by hand in photoshop).