dmg folders are stirred into ugly wrong layout

I'm running 10.8.4 on a MacBook. Whenever I download a new app, the dmg folder that traditionally has a "pretty" layout with stylized arrows showing to put the app into the App Folder is mixed up, with the graphics not appearing correctly. It doesn't affect functionality, and I know to put the app into the app folder which still works fine, so I just ignored it for months -- but it's annoying, and puzzling. Any idea how this problem could have originated, and what I can do to reverse it?