You may be too concerned about privacy and security to leave naked files lying around on a remote server. That's completely fair, but don't let it keep you from using the cloud. Boxcryptor is a free Windows desktop app that creates an encrypted folder that can be placed inside your cloud storage folder.
Once you create the folder and assign a password, simply drag and drop the files you want protected into that folder. Boxcryptor instantly encrypts and protects them using the AES-256 standard. To unlock the folder and view your files, simply run Boxcryptor, navigate to the encrypted folder and enter your password. For more details, see "How to encrypt your cloud storage for free."
If someone attempts to open the files without the password, an error message will show. However, only the file contents are hidden: The file name and file format are still in plain sight. So for super-duper extra security, change the file name to something innocuous.
It's worth pointing out that once you set the password for the encrypted folder, it can never be retrieved or reset. If you forget the password, the files inside the folder are lost forever. When choosing a password, choose something complex but unforgettable.
Developer Secomba offers editions for Android, iOS, Linux, Mac, and Windows RT as well. The free license is limited to one drive (and unlimited devices), but the $45 Unlimited Personal and $100 Unlimited Business licenses let you use as many computers and devices as you like. Secomba's comparison chart shows the differences between the different licenses.