Provide Fallback for Undisplayable Images #10

The user may have image files on their computer that they want PicPocket to manage but a browser can't display (e.g., raw images, editor files). It would be nice to have something display instead of a broken error link. related images might be useful (e.g., show exported file for an editor file) as might a pipeline for creating other formats of images (e.g., generate small samples for raw files).

Also worth considering: button to show in finder (mac-only feature, obviously).