I made a SqueakMap entry for the ImageFormat package. This is maintained
in the VMMaker repository but does not require VMMaker and may be of some
general interest for explaining the known image formats for Squeak that
are used with interpreter and Cog VMs.

ImageFormat represents the requirements of the image in terms of capabilities
that must be supported by the virtual machine. The image format version is
saved as an integer value in the header of an image file. When an image is
loaded, the virtual machine checks the image format version to determine
whether it is capable of supporting the requirements of that image.

Dave