If you really want to minimise the chances of unwanted access to your photos despite using someone else’s storage, you can always encrypt before syncing. I use duplicity, which uses GnuPG, to encrypt potentially sensitive information before sending data to remote locations.
However, I imagine the time overhead of encryption with a largish photography library could be rather inconvenient. As I consider it unlikely that anyone is going to be targeting my photography, unless I had photographs of a sensitive nature I probably wouldn’t bother encrypting before uploading to places like rsync.net.
Data uploaded to Spideroak is, I believe, encrypted and made unreadable, even to the provider, as part the automated process. (“Zero-knowledge”.) I do use Spideroak for some pre-encrypted work and personal data, and it’s been very easy to use so far.