Keep in mind too that all of the unlimited plans I've seen are for "personal-home use", not for commercial use, so using one to store your pictures would be violating their terms of service. May not be a big deal but if you upload 2TB of data that may draw attention to yourself and it wouldn't surprise me at all if they either asked you to pay extra or terminated your account.
Anything sold as unlimited isn't really unlimited, just unlimited for people who don't use it much
Doesn't hurt to try though, all you would lose is time.
I'm sure I've read of online backup solutions where you can send them the data on a physical media to get started, and order your data on physical media in the event of a data loss, I'm just not finding any right now. They'd probably be hundreds a month though for terabyte level of data.