Sorry, this is on a 2.53 GHz Mini with OS 10.6.8 and Preview 5.0.3 but what I read on the net, Apple discussion included, is that this is an ancient problem that has never been fixed by Apple in any version of software.
The only thing I can think of is to create the text in some word document where one can left or right justify text and then paste that as a picture into the main picture.
You might look at Seashore for this kind of thing.
I do use Seashore; I also use GIMP on occasion.
But the pictures open in preview and for something that simple, I expected annotations in Preview to be able to do this simple formatting.
With GIMP or Preview I have to launch yet another application, open the picture file in the application etc. - just a lot of extra work if one has a lot of pictures to do.
Well, I have used the annotation feature in Preview for years and the requirement to right justify an annotation just came up a few days ago.
Having it centred just doesn't work for that picture - I was a bit surprised that centre justified was chosen as the default and there seems to be no way to change that.
Every other application that handles text has that capability.