This couldn't be easier, just enter the URL,providing the URL points to an image of a supported type Mediawiki will automatically display the raw image.
Adding Images to Articles
The procedure for adding images to articles uses 'forward' or 'red' links and is preffered to the method of upload then link as it's less prone to error.
First enter your link preceded with the "Image" namespacelike so: [[Image:Myimage.jpg]] which will produce red link, click the link and it'll take you directly to the image upload page.
The link can contain additional formatting for images like: [[Image:ImageName.jpg|thumb|Caption]], more information can be found by following the links below.
Important: Image names are case specific. For example, if an image is called Picture.jpg then neither picture.jpg nor Picture.JPG will find it. File suffixes work in uppercase but doing so is bad practice.