1998.11.24 15:45 "Re: Photoshop Thumbnails", by Tom Lane
Look for IPTC - NAA (Information Interchange Model/Digital Newsphoto Parameter Record Version 4)
I believe Photoshop only writes the IPTC record if the user has explicitly added IPTC annotations to the file. I don't think that's where they keep the thumbnail, either.
Where they do keep the thumbnail (and a whole lot of other stuff) is a private Adobe tag that holds a block-structured list of items. They use this same block-structured add-on data in a whole bunch of different file formats --- for example, in a JFIF JPEG file they stick it into APP13 marker(s). Thus, they don't bother to support the file format's own native provision for thumbnails, if any.
You can find out a good deal about this topic in the Photoshop SDK documentation, available free from Adobe's website. The SDK fails to specify the contents of all the block types, however, and I think the thumbnail block is one of the things that could've been better documented. Expect to have to do some experimentation.
BTW I have a vague recollection that their JPEG-compressed thumbnails are broken --- IIRC the red and blue channels are swapped, at least in PS 4.
regards, tom lane
organizer, Independent JPEG Group