Image dimensions and zoom level are shown in the tab title.
Some sites require an anti-hotlinking workaround. You might see a progress bar and odd filenames when it's being used.
If videos don't play, check your content-blocking extensions and browser settings. Firefox users: Don't touch media.autoplay.enabled and be aware that Flashblock also blocks HTML5 videos by default.
The download feature is in an experimental state and not recommended for large files. "Save as" will not appear until the whole file has been loaded into memory. So don't hit the "d" key repeatedly. Press it once and wait.
If you want to disable this script on certain sites, add them to your script manager's exclude list. Firefox users: Open "Add-ons" -> "User Scripts" -> "Options" and write something like https://www.youtube.com/*.