TwentyFifteen:
- Fix image caption alignment in the editor.
- Remove unused var from functions.js.
Also includes few images that were shrunk a bit when running grunt precommit.
Fixes #30493.

git-svn-id: https://develop.svn.wordpress.org/trunk@30562 602fd350-edb4-49c9-b593-d223f7449a82