To refer to files on a page, use [[Image(filename)]] for images or [[File(filename)]] for general filenames where filename is one of the file names below.
Upload a new file
If you upload a file with the same name an an existing file then your version will replace the old version. If "Save as" is left blank, the original filename will be used (might be ugly) . You should give it a name! Just name it whatever.jpg/png/gif (in "Save as").
How do I do this?
Once you've selected a file on your hard disk, use "Save as" to name it whateveryouwant.png/jpg/gif. Then click "Upload" to upload the file to the page. To make an image appear on the page you need to edit the page and add the line [[Image(whatyounamedyourimage)]] where you want the image to appear. For general files, add the line [[File(whatyounamedyourfile)]] to make link to the file. That's it!