Not sure if this is possible, or if such an app exists, but I would like to try the following:

1. create an account on Amazon S3 storage. I don't want their consumer package because I feel it's over priced
2. use an app to automatically push photos from my phone to my storage space on Amazon
3. leave a thumbnail or marker image on the phone so I can still see what I have, but when I click the thumbnail, have the option of re-downloading back to the phone for viewing for 'x' period of time

Also, have the option of doing the same for my laptop files.

My understanding with Dropbox is it leaves a copy on your device and a backup copy on their systems, which defeats the purpose of keeping the actual file in the cloud and a placeholder on the device to reduce the amount of local storage taken up.

Having encryption on the cloud hosted files would be required as well.