Rename screenshot files, add descriptions

git-svn-id: https://plugins.svn.wordpress.org/pay2post/trunk@909394 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add description

git-svn-id: https://plugins.svn.wordpress.org/pay2post/trunk@909395 b8457f37-d9ea-0310-8a92-e5e31aec5664