changed image names

git-svn-id: https://plugins.svn.wordpress.org/web-simple-image-gallery/trunk@905432 b8457f37-d9ea-0310-8a92-e5e31aec5664
chagned image names for screenshots

git-svn-id: https://plugins.svn.wordpress.org/web-simple-image-gallery/trunk@905434 b8457f37-d9ea-0310-8a92-e5e31aec5664