This is exactly the problem Microsoft solved with OneDrive. You can choose what is kept only online and what is also offline. In windows at least its integration goes as far as keeping lower resolution versions of your photos offline for previewing purposes and seamlessly downloading the full-res when you try to open it in say Photoshop.

Unfortunately as nice as this sounds in practice its extremely slow (at least for us in Europe).

I am facing the same problem with hubiC which has a 10GB plan only and unless you have some crazy raid configuration on your desktop there is no way you can fill it up unless you use the web interface exclusively.