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The EXIF/XMP data is highly insightful, I didn't even think about how much space those would take up. I updated the original blog post with the information you provided here, and cited you for it. Hope you don't mind.

I mentioned in comments on your original article: I wasn't able to appreciably beat the BMW or portrait images.

Metadata would help explain this, since as a percentage of file size, it'd be a lot more detrimental on the first image at 45K than the BMW at 575K.

But sure seems there's something else going on with the first image's circular gradient; PS seems to be dithering its blocks in a moire pattern while GIMP is showing strong banding even on files w/o metadata.

Metadata is most certainly the case here. The breakdown of metadata for the original image (http://fc07.deviantart.net/fs21/f/2007/275/f/4/f4e91d2565442...):

  EXIF (APP1) -> 5328 bytes
  Photoshop IRB (APP13) -> 5896 bytes
  Adobe XMP (APP1) -> 26284 bytes
  Adobe (APP14) -> 12 bytes 
  Total -> 37520 bytes (36.6 kB)
That's 36 kB of metadata, leaving only 8 kB for compressed image data. This explains the banding seen in the English Hard post. The BMW image on the other hand (http://fc01.deviantart.net/fs51/f/2009/313/1/c/Jesus_Mobile_...):

  Photoshop Ducky (APP12) -> 15 bytes 
  Adobe (APP14) -> 12 bytes
  Total -> 27 bytes
The existence of the "Ducky" section means that the image was "saved for web" in Photoshop.

EDIT: Added numbers for the BMW image.

I got GIMP, and used the "Save as JPEG" output to see if it's including metadata.

According to jhead[1], GIMP is not including metadata in the saved JPEG.

At quality "13", the file was 46302 bytes. Using the "purejpg" option on jhead, it dropped to 45592, or -710 bytes.

This 45592 bytes is exactly, to the byte, what the original article found.

[1] http://www.sentex.net/~mwandel/jhead/usage.html

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