The 16px file icons need some love to make them more recognisable and to make them fit in with the other icons on the course page and elsewhere.

I suggest:

1) The generic image icon should not be a document (at any size, really). Make it a generic photo with a border, something like http://www.iconfinder.com/icondetails/7366/128/image_photo_style_icon

2) If the icon is a document with a shape in the middle, then throw away the document border and just make the center shape 16x16. So less like http://www.iconfinder.com/icondetails/47975/128/audio_file_icon and more like http://www.iconfinder.com/icondetails/42341/48/high_sound_speaker_volume_icon , for example.

3) Tweak the image manually to make it crisper on screen.