In the Twenty Twelve theme the text for an image caption defaults to the left. I have several questions:
1) what is the code to center the text?
2) what is the code to start the text from the right?
3) can these be done by post or is it theme wide?
The caption text in the posts won’t accept any CSS as far as I know. I think there was talk of letting in a little HTML so people could add links to image sources for attribution, but I don’t know if that was ever done.
The following, added to the bottom of your custom CSS, will allow you to either center, or right align the text. To right align, just replace “center” with “right”.