By going to the Main Page and clicking the 'add image' link in the sidebar, on the right, directly under the menus and above the 'random page' link. This will take you to a screen which will invite you to browse your hard-drive for the file you like. Then you can either give it a new filename for the Wiki or not, as you like, and give a description for it. And then click 'upload file' and you're all done!