I'd use this format:
rsync -avze ssh --delete 'a:\pictures' 'USER@backup.dreamhost.com:pictures'
a = all
v = verbose
z = compressions
e = shell to use
-delete = mirror, so it deletes destination files that you don't have locally
For a dry run, use rsync -avzne
The 'n' makes the first command just pretend it's working.