Just to state some obvious things.....
It would have to include all of these:
1. What guns and how much ammo you had for each.
2. What enemies you've alredy killed and where the ones that are left are.
3. You already said all the image files and all that stuff.
4. Which gun you had out at the time of the save.
5. Which objectives you have already completed.
6. All the items you had.
7. Of course, which level you are on.
8. All of the secrets that you've found.
9. All of the things in the level that you have picked up and where the ones that you haven't are.
10. The force powers that you have and at what level they are all at.
11. Where any object that you have destroyed or moved currently is.
12. Which switches you have activated and which ones you haven't. What doors are open/unlocked and which ones aren't.
13. Where objects that are on a fixed moving path currently are.
14. How much health and shieldage you have left.
Anyways, these are all just common sense becasue you couldn't start just as you were before if it didn't know these things.
Hope some of this helps, you may have already thought of most of it, but just laying it out there.
If you are looking for technical stuff, then i can't help you a whole lot and you can completely ignore this post. If that is the case, sorry.