You need an online copy - no need to use RAID or anything particularly fancy for it - just buy as large a drive (or drives) as you need, connect to one of your computers and then share them so that other computers on the network can access them.
For protection, you need to keep at least one (and preferably two) additional OFFLINE copies. DVDs work fine, hard drives do too. Hard drives are a lot easier to deal with because you don't need to manually swap media.
Hard drives can fail, but so can DVDs. To protect your data the choice of media is a lot less important than having multiple copies. And for the very best protection at least one copy should be OFFSITE.
RAID is NOT a good solution for archival protection of your photos because it can ONLY protect against disk drive failure and not against the many other risks (accidental deletion, corruption, theft of the computer system, etc. etc.)
The best reason to use RAID is to protect against downtime due to drive failure - if your business will loose money during the time you can't access the photos due to a drive failure then that's an excellent reason to use RAID.