Good for you for thinking about this BEFORE your drive dies! I've seen so many posts about how to recover cherished data from dead drives, and unfortunately by that time it's usually too late...
Having your pictures on a different drive than the OS means they're OK if the OS drive dies - but if the data drive dies then you'll still loose them. ANY disk can fail, not just the OS disk.
At the VERY LEAST you should put a copy of them on the OS disk too. But I'd strongly encourage you to buy an external drive and put a copy of them there. You might want to burn them to DVDs too.
Good backup strategy says that in addition to the data stored on your computer, you should also have two offline copies of your data, one of which is stored offsite. It's not complicated, you don't have to worry too much about HOW you make your copies (DVD or external USB or back up to an internet hosting service), but if you follow this strategy then you're covered for almost any conceivable scenario.