My usual disclaimer: I'm not an expert at anything.
1. The reserve partition is usually 100MB. It contains the bootmgr which is what helps your computer to boot. It's called an active partition because without it your computer is just an expensive door stop.
2. The answer to this really depends on which imaging tool you're going to use. For example, if you use the built in Windows 7 imaging tool it will automatically image everything on your hard drive. When it comes time to use that image it will automatically replace everything on your hard drive. But if you use another imaging tool like Macrium free, you can select which partition(s) to image. A lot of people just image the C: drive because that's what changes over time. The reserve partition shouldn't change. But it wouldn't hurt to image the reserve partition "just in case".
3. As above, it should be OK to just image the C: drive. I'm not sure what you mean when you say you don't have a system partition. Did you mean the system reserve partition? If so, as long as your computer is working properly, no worries.
4. There should be no need to maintain the reserve partition other than perhaps making an image and storing it safely some place (like an external hard drive.) In fact, I've heard stories that changing the reserve partition by increasing or decreasing partiton size, etc can cause major problems.
If I've made any errors in what I've said I'm sure other more knowledgeable forum members will correct me.
EDIT: whs beat me to it. He taught me everything I know and he's faster on the keyboard than me!