standard images when the learning objects have no thumbnails

app/assets/images/animation.png

app/assets/images/audio.png

app/assets/images/game.png

app/assets/images/image.png

app/assets/images/link.png

app/assets/images/simulator.png

app/assets/images/software.png

app/assets/images/text.png

app/assets/images/video.png

app/assets/stylesheets/application/learning_objects.scss

app/helpers/learning_objects_helper.rb

app/repositories/orient_db/learning_object_repository.rb