Note this is somewhat outdated, but mostly true. See WN:IUP and WN:FU for more up to date information. 
In order to be able to use a file in Wikinews, you must upload it to the Wikimedia Commons first. Create a Commons account, if you haven't already, upload the file, and use it here using the syntax [[File:Filename]].
Please note that only files under a free license, such as the GNU Free Documentation License or the Creative Commons Share-Alike license, can be used on Wikinews. Fair use of images is not allowed because we are directly competing with other news media, and the associated legal risks are much larger than on other projects like Wikipedia.
Wikinews:Upload media