Blank page fix - Only open image with WP_Image_Editor if the required size does not exist on disk

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@771570 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update thumbnail styling

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@771571 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update thumbnail styling

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@771572 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update thumbnail styling

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@771573 b8457f37-d9ea-0310-8a92-e5e31aec5664