The first 64 bytes of the file contain the text description that was entered when the image was created. These can essentially be ignored for disk translation purposes.
The next 624 bytes are unknown to me.
The next 400 bytes contain the starting locations for each track in the disk image file. Each pair is the address shifted right by 8 bits. The sides are interleaved so that track 0 side 1 comes right after the pair for track 0 side 0.
The next 400 bytes contain the lengths of each track. These are in the same format as the above.
There seem to be 3 more blocks of 400 bytes that follow, that I cannot figure out. They don't seem to be important though.
After these blocks the track data follows. The tracks are simple bit-images so they need to be read in (as the Disk II hardware does), and converted into their associated bytes before they can be used in an emulator.
In short, the image looks like this:
64 byte header
224 byte block
400 byte block
400 byte block of starting addresses
400 byte block
of track lengths
400 byte block
400 byte block
400 byte block
400 byte block
Track data
My current listing for the image file is available. for those who wish to look at it. However, please remember that the source is commented so that I can understand it. Also, I haven't worked on this in quite a while, part of the reason I'm making it available is so that others can continue working on it.