It is generally a good idea to upload files/images "to a page" rather than "to the swiki" (just to keep from cluttering up the top level).
So what do you do if you upload a file to the wrong page (or want to refer to a file you've already uploaded to another page).
Certainly you could upload the file again. However, it is pretty straightforward to refer to a file on another page.
Go to the page that you uploaded the file to and edit it. The file plugin syntax for a file on a given page looks like this: <?file src="filename.doc"?>, right? If you are moving the reference, you can cut that part on the current page and make a note of the page number (the page number is shown in the url in the address bar of your browser–for THIS page it is 21).
Now go to the page you want to put the file reference into and edit it. Paste the file plugin syntax you cut from the other page and add in a page=pagenum option: <?file src="filename.doc" page=21?>.