You could also back up to something like AWS Glacier. The cheapest tier (access less than once a year) is $1/TB/month. Maybe if you kept thumbnails locally, you could push all the data up and only pull it as and when you needed it.
Aaaah I see : - ) still, if it's only very occasional "do you remember that photo?" queries, that shouldn't add up to any significant cost. But a full retrieval - yeah. Interesting how the price ramps up!
