I just realized that I already have an encrypted volume stored on Dropbox, so I did not need to ask. But to answer yours: I'm not 100% sure about iCloud, but a

I just realized that I already have an encrypted volume stored on Dropbox, so I did not need to ask. But to answer yours: I'm not 100% sure about iCloud, but at least Dropbox seems to have no trouble uploading only the changes of my (large) encrypted volume, whether it's mounted or not. I have run into problems, however, when I mount the image on multiple computers, so I'd recommend against that, or at least making any changes before everything is properly synced.