Well, the original images and 100% crops are indistinguishable to my eye.
Here's why:
JPEG image compression FAQ
http://www.faqs.org/faqs/jpeg-faq/part1/
By Tom Lane
March 1999
Subject: [10] Does loss accumulate with repeated compression/decompression?
It would be nice if, having compressed an image with JPEG, you could
decompress it, manipulate it (crop off a border, say), and recompress it
without any further image degradation beyond what you lost initially.
Unfortunately THIS IS NOT THE CASE. In general, recompressing an altered
image loses more information. Hence it's important to minimize the number
of generations of JPEG compression between initial and final versions of an
image. . .
It turns out that if you decompress and recompress an image at the same
quality setting first used, relatively little further degradation occurs.
This means that you can make local modifications to a JPEG image without
material degradation of other areas of the image. (The areas you change
will still degrade, however.) Counterintuitively, this works better the
lower the quality setting. But you must use *exactly* the same setting,
or all bets are off. Also, the decompressed image must be saved in a
full-color format; if you do something like JPEG=>GIF=>JPEG, the color
quantization step loses lots of information. . .
The bottom line is that JPEG is a useful format for compact storage and
transmission of images, but you don't want to use it as an intermediate
format for sequences of image manipulation steps. Use a lossless 24-bit
format (PNG, TIFF, PPM, etc) while working on the image, then JPEG it when
you are ready to file it away or send it out on the net. If you expect to
edit your image again in the future, keep a lossless master copy to work
from. The JPEG you put up on your Web site should be a derived copy, not
your editing master.
I can not see any degradation after 80 re-saves. At least none that would worry me.
