I want to do exactly the same with my photos and personal videos too, although i use crash plan aswell but i became paranoid after loosing some photos accidentally, so more straightforward way to do this in OneDrive is-[I use a mac]
Put the photos you want to back up on OneDrive in the pictures folder as usual(all setting being default) and wait for the photos to be uploaded.
After upload is complete, open OneDrive in your browser then create a new folder there- doing this will make that folder appear on your computer as well, Now go to 'oneDrive'---> 'Preference'-->'Choose folders' and uncheck this newly created folder(just this folder so you can use onedrive as usual), Now go back to browser and transfer your uploaded photos to this folder...
double check them that they are all there and delete them from the HDD of your computer...
from now on all you have to do each time you want to save new photos on OneDrive( onlineOnly) is- put them in the photos folder(or in any other folder except the new folder you disabled sync for) open one drive and move them to the unsynced folder and delete them from your hdd..
This is a more of a work around of course..