For all your questions about jpeg, try here:
http://www.faqs.org/faqs/jpeg-faq/part1/ e.g.: Subject: [10] Does loss accumulate with repeated compression/decompression? TL> In general, recompressing an altered image loses more information. TL> It turns out that if you decompress and recompress an image at the same TL> quality setting first used, relatively little further degradation occurs. TL> This means that you can make local modifications to a JPEG image without TL> material degradation of other areas of the image. (The areas you change TL> will still degrade, however.) Counterintuitively, this works better the TL> lower the quality setting. But you must use *exactly* the same setting, TL> or all bets are off. TL> Unfortunately, cropping doesn't count as a local change! JPEG processes TL> the image in small blocks, and cropping usually moves the block boundaries, TL> so that the image looks completely different to JPEG. You can take TL> advantage of the low-degradation behavior if you are careful to crop the TL> top and left margins only by a multiple of the block size (typically 16 TL> pixels), so that the remaining blocks start in the same places. (True TL> lossless cropping is possible under the same restrictions about where to TL> crop, but again this requires specialized software.) And: Subject: [13] Isn't there a lossless JPEG? [...] TL> because there are several different compression methods all known as "JPEG". TL> "baseline JPEG" (or its variant "progressive JPEG"). The same ISO TL> standard also defines a very different method called "lossless TL> JPEG". And if that's not confusing enough, a new lossless standard TL> called "JPEG-LS" is about to hit the streets. TL> Lossless JPEG is a completely different method that really is lossless. TL> However, it doesn't compress nearly as well as baseline JPEG; it typically TL> can compress full-color data by around 2:1. TL> Lossless JPEG has never been popular --- in fact, no common applications TL> support it --- and it is now largely obsolete. (For example, the new PNG TL> Recognizing this, the ISO JPEG committee recently finished an TL> all-new lossless compression standard called JPEG-LS standard TL> outcompresses lossless JPEG on most images.) So, I might have been partially wrong about the "resave at same quality setting" not affecting the photo. The FAQ is not clear on it to me... Frantisek
