The directory was always images. As for other changes I don't know of
any listing of all the changes (you could of course browse through the
changelogs). I've ported a number of apps over from 1.1.x to 1.2.x
and not run into any significant changes. There were a few things
marked deprecated that I ran into like start_form_tag/end_form_tag,
but those were easy to clean up and didn't break anything.
1.2.2 had an issue with functional tests needing strings instead of
symbols for the session hash, but 1.2.3 fixed that.