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Normally, I would suggest
Flickr - which gives you 1TB of free space to store images. But Flickr is owned by Yahoo!, which isn't doing so well financially, and recently announced they would be scaling back development and support for many of their services. I doubt they would outright kill the Flickr service, but who knows at this point.
Outside of Yahoo! owning it, Flickr is a fantastic service.
The problem with "FREE" is that it WILL go away eventually. Copy/CudaDrive was free, and from a much larger publicly traded company than BOX. I had nearly 2TB of free space from referrals, and stored everything on it—I mean absolutely everything on my Mac was in the Copy folder (thus synced to all my Apple devices with no effort on my part). I loved the service, and even paid for a pro plan at my office (even though I didn't need the space).
Barracuda Networks just announced that they are cancelling the entire service in a month. Poof.
BOX is in a race to the bottom with Dropbox to see which one can survive against Google and Apple (I won't bother with Microsoft's services, because nobody else does anymore, either). Dropbox has a ton more users than BOX, but BOX has paying users in enterprise. Unfortunately, they don't have nearly enough paying users to survive.