It's not the most elegant add-on, but the first I found that actually does the trick. You'll have to open the video or picture in a new tab for it to work, it doesn't work in the overlay viewer. Then, the links will appear in a frame in the top right, as shown in the screenshot. It doesn't directly initiate the download, but sets the browser location to the actual image or video file. However, many people might prefer that it works this way. It directs you to the full version of the pictures (the web interface often shows downscaled copies) and even works for multi-picture posts. Great work! An option to make it also work in the overlay viewer would make it a 5-star extension.