The article asks:
I was preparing a book for one of my clients and as I am uploading the photos, which are personal, the first thought was... should I really be uploading these photos to this website, we just met?
The answer is no, of course you shouldn't trust any website. If you want it to remain private, leave it off the internet.
I assume that whoever is speaking in the article has a job / contract to prepare these photos for clients who have requested that they upload the photos to the service. In that case leaving them off the Internet is not an option.