I have my photo collection synced to Ubuntu One and I'm wondering what will happen if I use Shotwell/F-Spot to add tags to my photos (by default those apps seem not to write metadata changes to the files themselves, but I've changed that). Will the changed files be uploaded again or is UbuntuOne smart enough to upload just the changes?
I do this all the time with photos from my mobile phone, and the entire photos get re-uploaded each time you change anything. I usually crop and tag the photos when I'm back in front of the computer, and each time I save a change the entire file uploads again.
Side note: The original photos shot on the mobile don't get updated, so I periodically wipe them from the phone's memory and copy the edited ones back to it from the computer. If you use the Ubuntu One Files app for Android be aware of this.