@urcindalo welcome to the Rockstor community!
There are probably many differing opinions out there, especially what a “tight” budget means 
Here is my take, but don’t take that as the end-all-be-all. Fundamentally, since these are your precious pictures, I am not sure whether you have other backup methods external to your current setup, or is your current 2TB drive 1 the “operating” drive, and you use the second 2TB drive as a backup that you store separate from your working system (since, if your system goes up in flames you would still lose both drives).
I will make some other assumptions as part of my opinion:
- you will already have some type of smallish SSD drive available to run the Rockstor OS on.
Instead of buying a 4TB SSD, you could purchase 2 x 4TB HDDs probably for the same total amount of money (e.g., a 4TB WD Red for USD79 each vs. USD140-250 for a single 4TB SSD in the U.S., your mileage may vary in a different part of the world, of course).
You could then:
- put the 2x 2TB and 1x4TB into your Rockstor box
- create Pool A from those 3 drives
- set up a btrfs RAID1 on the pool, which should give you 4TB storage with some redundancy
- Use the second 4TB in an external case, or in a physically separate system dedicated to backups and set up your luckyBackup software (or rsync, or what have you) to periodically back up your pictures onto the HDD external to your daily usage Rockstor system.
That way you essentially have 3 copies floating around for your pictures, which should make it reasonably save. And you have “size parity” between your Rockstor storage available and your backup drive. Of course, if you can set up the second 4TB drive as an external backup drive that you can store off-site to further minimize the risk and a good backup schedule … you get the picture.
One thing I would ensure is that it’s worth to continue to rely on those 2TB drives. The S.M.A.R.T. statistics will give you some indication, though not foolproof. Of course, depending on how “old” these are, there will also be a difference in read-write speed compared to the newest incarnation of drives. Good part is, you can start with a setup like this, when your budget improves down the road, you could start adding another 4TB drive, further increasing your storage, and/or replacing the 2TB drives. But I would ensure, that as your Rockstor capacity grows, your backup capacity does likewise. For example:
in 2 years buy a 6TB drive, move the backup 4TB to your existing pool (gain 6TB Rockstor storage space) and use the 6TB as your new backup drive …
As usual, in these case, the answer is really “it depends”, but the one thing I will emphasize is that Rockstor by itself is not a backup solution, so, especially for your pictures, a corresponding backup setup is almost equally important.
Hope, that gives you some further ideas.