To start with let's look at the "raw" Voyager 1 & 2 image archives. These are under VG_0xxx, and there's a "file by file" view at http://pds-rings.seti.org/vol/VG_0xxx/.
Each compressed volume contains the same layout - at the top level the overview document AAREADME.TXT describes the file layout and content, and the DOCUMENT/VOLINFO.TXT describes the data format.
Breaking down what appears where it's:
- Uranus on 1-3
- Subset of Saturn on 4-5
- Subset of Jupiter on 6-8
- Neptune 9-12
- All of Jupiter 13-25
- All of Saturn on 26-38
For a starter I'll pull down C4394416.IMQ from VG_0005 as a reference file for the next bit of work, which will be to extract the basic file structure and header information.