If you convert the gsm file with the XML Converter Tool to xml, you can find the binary part of the preview picture.
It starts with the XML-Tag like this:
<Picture MIME=”image/png” SectVersion=”19″ SectionFlags=”0″ SubIdent=”0″ length_in_bytes=”9564″ platform=”Win”>
SubIndent = “0” is always used for the preview picture.
You can embedd more pictures into the object with use of the XML Converter Tool and use those pictures for example in the User Interface, but as far as I know you cannot change the selection of the preview to another of the embedded pictures.