On the left hand side there is a "toolbox" section, within that there is an
Upload file option. This lets you pick the image from your computer and upload it. Once it's been uploaded it'll show you a new page and the title will be
File:YourFileName.jpg (depending on the type of file) Keep a note of this as this is how you'll include it on a page.
To use your image on any page just put [[File:YourFileName.jpg]] and the image will be included.