Added image file settings, admin panel

Admin settings added to File settings panel:
Maximum preview image width (in pixels, optional) - if not set, will default to current 200px hardcoded setting
Maximum preview image height (in pixels, optional) - if not set, will default to current 200px hardcoded setting
Hide file info (name, size) for images on wall (defaults to current show info setting)

Added css, slight padding between preview images (images look better if larger, but stacked).