Allow jpg's for screenshots too. props downstairsdev

git-svn-id: http://plugins.svn.wordpress.org/theme-check@903528 b8457f37-d9ea-0310-8a92-e5e31aec5664
add banner images

git-svn-id: http://plugins.svn.wordpress.org/theme-check@904291 b8457f37-d9ea-0310-8a92-e5e31aec5664
rm text in banner 

git-svn-id: http://plugins.svn.wordpress.org/theme-check@904294 b8457f37-d9ea-0310-8a92-e5e31aec5664