Rongnongno, is 100 percent correct. I work mostly with JPEG, and will tell you, They are downgraded right out of the camera. To what degree is the question. I know there changed from 16 bit to 8 bit if that means anything. I will go onto say that in general you will not see the loss visually for at least like the first 10 saves of the same file. Take a JPEG, open it than save it. Photo1.1 then save it again as Photo1.2. Open it and save it as Photo1.3. Open it again and again. It will considerable closing to see any change. If you blow it up these changes will appear somewhat faster. I would recommend that if you are going to be working on a photo for any period of opening and closing that you SAVE as a DNG, TIF or whatever native format that chose that doesn't use compression. This will maintain the photo quality till the end. Or try to refrain from saving the original.

defrag doesn't just sit on the drive, the file is opened and combined into another spot on the drive. In defrag, as I am sure you understand being a computer engineer (this is what my electrical engineering degree is in) the drive has written bits of the software to various open spots across the drive and not necessarily in sequence. Defrag moves all of the bits and pieces to an open space creating a large open space at the beginning of the drive then moves the data again in order to fill the drive consecutively. This is at least 2 opens and saves usually more.

I was looking at some photographs I took 6/10 years ago, that came out of a point and shoot at around 6mp, now are less than a 1k-kbs {between 800kbs to 300kbs}, I am pissed.. I do not think there is any way to restore, some were pretty good, they could have been worked with, now they don't even look good on the computer screen, @ about 50% they are pixelated!! there crap!!
I know everyone has there own way of saving files! I have recently learned to take the photo JUST in RAW, work with that and save , now I only make a JPG if I am posting to web site, then delete it from my library,
I remind you, that is just the way I am doing it, and when I learn a better way I will go with that ..

good luck

defrag doesn't just sit on the drive, the file is opened and combined into another spot on the drive. In defrag, as I am sure you understand being a computer engineer (this is what my electrical engineering degree is in) the drive has written bits of the software to various open spots across the drive and not necessarily in sequence. Defrag moves all of the bits and pieces to an open space creating a large open space at the beginning of the drive then moves the data again in order to fill the drive consecutively. This is at least 2 opens and saves usually more. In antivirus, the file is opened and examined by Norton, AVG or what ever and examined for strings that are equivalent to strings of data in a virus and then either resaved, saved in a vault for further review or deleted. This is why the process of running a virus scan takes so long. Watch a read/write program on your computer as the virus scan works. The reads and writes will jump dramatically showing the computer is actually opening then rewriting the data.

Your best bet with any images, but especially jpg is to backup to DVD or CD and put it away until you need it.