I'd like to suggest https://www.pixelapse.com
It was made for designers with designer's needs in mind. Other solutions (GIT, SVN, etc.) cannot give you proper usability, and as well, your clients would be not able to comment, review and browse design milestones in an easy way.
Other way is to use https://layervault.com but they are struggling with some security issues. Also the usability is not really great.
