Missing image file previews with Media
Are you saying that the thumbnail images for file uploads aren't displaying (a broken image icon the browser)?

If so, depending on how you installed Drupal ... drupal (well your webserver) needs to own the /sites/default/files/styles directory, or else drupal cannot create thumbnails for preview on-the-fly.

This is a known Issue: theme_image_style_preview() and image_requirements() should check whether style directories are writable.