InstantCommons is a MediaWiki feature that allows an article to use media files hosted by Wikimedia Commons. If an article refers to an inexistant file, MediaWiki will automatically look for a file under the same name in Commons. If the file is located, it will be downloaded and displayed in the article, as well as cached locally for future use.
This allows you to include multimedia contents in your articles as long as that content is under a free license (Commons requirement). On the other hand, backups for this wiki can be kept small because images downloaded from Commons using this feature can be safely scrapped and do not add to backup size.
If you must use an image, PDF file, etc. in your article, follow these steps.
Using content already available in Commons
1. In Commons, navigate to the file you want to use. The interesting bit is the name after the File: part, e.g. in "<URL>File:Foo.JPG" the identifier Foo.JPG is all you need.
2. Following this example, point to "Foo.JPG" in your article as you would with a local file, bearing in mind that case is important (foo.JPG != Foo.JPG != Foo.jpg). The file will be displayed and handled as any regular file that was uploaded locally. The license is also downloaded automatically.
Using content not (yet) available in Commons
Hotlinking external files without permission in an arbitrary fashion is generally a bad idea (not to mention bandwidth theft) and therefore discouraged, so please do NOT do it. If you are the owner of a media file that you want to use in this wiki and you're willing to license it under a free license, the procedure is simple:
1. Point you browser to Wikimedia Commons
2. Register/login and upload your file there, specifying a suitable license and probably one or more categories.
3. After the media file becomes publicly available, just refer to its name in your article here as explained above.