The upload bookmarklet lets you upload images to Danbooru in a single click.
To install it, drag the "Post to Danbooru" link above to your browser's bookmarks toolbar (beneath the address bar). If your bookmarks toolbar isn't visible, press Ctrl+Shift+B to show it, or look for the "Show bookmarks toolbar" option in your browser's Bookmarks menu.
To use it, simply click the "Post to Danbooru" link in your toolbar when viewing an image. You can use it on either the image itself, or on the page containing the image.
You can use the upload bookmarklet on any of the sites below: